He was a Sinn Féin councillor on Dublin Corporation from 1909 until 1922 and joined the Irish Volunteers in 1913. Cosgrave played an active role in the Easter Rising of 1916 serving under Eamonn Ceannt at the South Dublin Union. Following the rebellion Cosgrave was sentenced to death, however this was later commuted to penal servitude for life and he was interned in Frongoch, Wales. While in prison Cosgrave won a seat for Sinn Féin in the 1917 Kilkenny by-election.

He again won an Irish seat in the 1918 general election, serving as MP for Carlow – Kilkenny. He was released from prison in 1918 under a general amnesty and took part in the soon to be established Dáil Éireann. On 24 June 1919 Cosgrave married Louisa Flanagan in Dublin.

Sinn Féin proved to be the big winner of the election in Ireland, capturing 73 Irish seats, 25 uncontested. Its manifesto promised abstentionism from the House of Commons in Westminster. On 21 January 1919, Sinn Féin's MPs who were not imprisoned assembled in the Round Room of the Mansion House in Dublin and formed themselves into an Assembly of Ireland, known in the Irish language as Dáil Éireann. Cathal Brugha became Príomh Aire (First or Prime Minister), also called President of Dáil Éireann.

In April 1919 Brugha resigned and Éamon de Valera, the Sinn Féin leader, who had just escaped from prison with the help of Michael Collins using a key made from a candle, assumed the premiership instead. The new government and state, known as the Irish Republic, claimed a right to govern the island of Ireland. It also declared UDI, that is, a declaration of independence which remained until the end of the Republic unrecognised by any other world state except the Russian Republic under Lenin.

Though one of the most politically experienced of Sinn Féin's MPs (by now called Teachtaí Dála), Cosgrave was not among the major leadership of the party. Nevertheless he was appointed to de Valera's cabinet as Minister for Local Government, his close friendship with de Valera being one of the reasons he was chosen. Another reason was his long time experience gained on Dublin Corporation, most recently as Chairman of its Finance Committee. His chief task as minister was the job of organising the non-cooperation of the people with the British authorities and establishing an alternative system of government. Cosgrave was very successful in his role at the Department of Local Government.

In 1920 he oversaw elections to local councils in which the new system of proportional representation was used. Sinn Féin gained control of 28 of the 33 local councils. These councils then cut their links to the British, and pledged loyalty to the Sinn Féin Department of Local Government, under Cosgrave.

Cosgrave broke with Éamon de Valera over the issue of the Anglo - Irish Treaty of 1921. To de Valera and almost half of the Sinn Féin TDs, the treaty betrayed "the republic" by proposing to replace it with dominion status akin to the position of Canada or Australia within the British Empire.

To a majority, however, republican status remained for the moment an unattainable goal, with the republic unrecognised internationally. Dominion status offered, in the words of Michael Collins "the freedom to achieve freedom." At the cabinet meeting in Dublin held to consider the Treaty immediately after it had been signed, Cosgrave surprised de Valera by agreeing with Collins and with Arthur Griffith, de Valera's predecessor as leader of Sinn Féin and the chairman of the delegation which included Collins that had negotiated the Treaty. After the Dáil voted by 64 to 57 to approve the Treaty in January, 1922, De Valera resigned the presidency (which in August 1922 had been upgraded from a prime ministerial President of Dáil Éireann to a full head of state, called President of the Irish Republic). De Valera was replaced as president by Griffith. Collins, in accordance with the Treaty, formed a Provisional Government which included Cosgrave.

The months following the acceptance of the Treaty saw a gradual progression to civil war. The split in Sinn Féin gradually deepened and the majority of the IRA hardened against accepting anything less than a full republic. Collins and de Valera tried desperately to find a middle course and formed a pact whereby Sinn Féin fought a General Election in June with a common slate of candidates. Despite this pact, the electorate voted heavily in favour of pro-Treaty parties. On the day of the election, the draft Free State Constitution was published and rejected by the Anti - Treatyites as it was clearly not a republican document. Collins, forced to a decision, opted to maintain the Treaty position and the support of the British Government, and moved to suppress the Republican opposition that had seized the Four Courts in Dublin. The Civil War started on 28 June 1922, and the IRA was decisively defeated in the field over the following two months, being largely pinned back to Munster. In August 1922, both Griffith and Collins died in quick succession; the former of natural causes, the latter a few days later when ambushed by Republicans at Béal na mBláth. With de Valera now on the fringes as the leader of the Anti - Treaty forces in the Civil War, the new dominion (which was in the process of being created but which would not legally come into being until December 1922) had lost all its most senior figures.

Though it had the option of going for General Richard Mulcahy, Collins' successor as Commander - in - Chief of the National Army, the pro-Treaty leadership opted for Cosgrave, in part due to his democratic credentials as a long time politician. Having previously held the Local Government and Finance portfolios, he now became simultaneously President of Dáil Éireann (Griffith had returned his office to its pre-August 1922 name) and Chairman of the Provisional Government. When, on 6 December 1922, the Irish Free State came into being, Cosgrave became its first prime minister, called President of the Executive Council.

W.T. Cosgrave was a small, quiet man, and at 42 was the oldest member of the Cabinet. He had not sought the leadership of the new country but once it was his he made good use of it. One of his chief priorities was to hold the new country together and to prove that the Irish could govern themselves. Some historians have noted that he lacked vision as a leader and was surrounded by men who were more capable than himself. However, over his ten years as President he provided the emerging Irish state with an able leader who had a sound judgement on the matters of state that the new country was facing.

As head of the Free State government during the Civil War, he was ruthless in what he saw as defence of the state against his former republican comrades. Although he actually disagreed with the use of the death penalty in principle, in October 1922 he enacted a Public Safety Bill, which allowed for the execution of anyone who was captured bearing arms against the state or aiding armed attacks on state forces. He told the Dáil on 27 September 1922, "although I have always objected to the death penalty, there is no other way that I know of in which ordered conditions can be restored in this country, or any security obtained for our troops, or to give our troops any confidence in us as a government". Cosgrave's position was that a guerrilla war could drag on indefinitely, making the achievement of law and order and establishing the Free State impossible, if harsh action was not taken. His reputation suffered after he ordered the execution without trial of republican prisoners during the civil war. In all 77 republicans were executed by the Free State between November 1922 and the end of the war in May 1923, including Robert Erskine Childers, Liam Mellowes and Rory O'Connor, far more than the 14 IRA Volunteers the British executed in the War of Independence. The Republican side, for their part, attacked pro-Treaty politicians and their homes and families. Cosgrave's family home was burned down by Anti - Treaty fighters and an uncle of his was shot dead. Cosgrave said " I am not going to hesitate if the country is to live, and if we have to exterminate ten thousand Republicans, the three million of our people is greater than this ten thousand". In April 1923 the Pro-Treaty Sinn Féin members organised a new political party called Cumann na nGaedheal with Cosgrave as leader. The following month the civil war was brought to an end, when the remaining Anti - Treaty IRA guerrillas announced a ceasefire and dumped their arms.

In the first few years in power Cosgrave's new government faced a number of problems. Firstly, the government attempted to reduce the size of the Irish Army. During the civil war it had grown to over 55,000 men which, now that the civil war was over, was far too large and costly to maintain. However, some army officers challenged the authority of the government to cut the size of the Army. The officers, mostly Pro - Treaty IRA men, were angry that the government was not doing enough to help to create a republic and also that there would be massive unemployment.

In March 1924 more layoffs were expected and the army officers, Major - General Liam Tobin and Colonel Charles Dalton sent an ultimatum to the government demanding an end to the demobilisation. Kevin O'Higgins, the Minister for Justice, who was also acting President for Cosgrave while the latter was in hospital, moved to resolve the so-called "Army Mutiny". Richard Mulcahy, the Minister for Defence, resigned and O'Higgins was victorious in a very public power struggle within Cumann na nGaedheal. The crisis within the army was solved but the government was divided.

In 1924 the British and Irish governments agreed to attend the "Boundary Commission" to redraw the border which partitioned Ireland between the Irish Free State and Northern Ireland. The Free State's representative was Eoin MacNeill, a respected scholar and Minister for Education. The Free State expected to gain much territory in heavily Catholic and republican parts of counties Londonderry, Fermanagh, Tyrone, and Armagh, as the British government had indicated during the treaty negotiations that the wishes of the nationalist inhabitants along the border would be taken into account. However, after months of secret negotiations a newspaper reported that there would be little change to the border and the Free State would actually lose territory in Donegal. MacNeill resigned from the commission and the government for not reporting to Cosgrave on the details of the commission. Cosgrave immediately went to London for a meeting with the British Prime Minister and the Prime Minister of Northern Ireland, where they agreed to let the border remain as it was, and in return the Free State did not have to pay its pro-rata share of the Imperial debt. In the Dáil debate on 7 December Cosgrave stated: "I had only one figure in my mind and that was a huge nought. That was the figure I strove to get, and I got it."

Cosgrave notably turned down a plea for asylum in Ireland for Leon Trotsky while in exile. The request was made by the trade union leader William X. O'Brien in 1930. Cosgrave told O'Brien

Told him [O'Brien] "I could see no reason why Trotsky should be considered by us. Russian bonds had been practically confiscated. He said there was to be consideration of them. I said it was not by Trotsky, whose policy was the reverse. I asked his nationality. Reply Jew. They were against religion (he said that was modified). I said not by Trotsky. He said he had hoped there would be an asylum here as in England for all. I agreed that under normal conditions, which we had not here, that would be alright. But we had no touch with this man or his Government, nor did they interest themselves in us in his 'day'.

In June, 1927, a general election was held in which de Valera's new party, Fianna Fáil, won many seats on an abstentionist platform. In July, the Minister for Justice, Kevin O'Higgins, was assassinated on his way home from Sunday Mass by the IRA. Cosgrave had legislation passed to force Fianna Fáil to take their seats in the Dáil and this proved successful with de Valera and his party entering the Dáil. Previously, without de Valera, Cosgrave faced very little opposition, giving him considerable freedom of action. However, de Valera's arrival significantly altered the situation.

Although Cosgrave and his government accepted dominion status for the Irish Free State, they did not trust the British to respect this new independence. The government embarked on fairly radical foreign initiatives. In 1923 the Irish Free State became a member of the League of Nations. The Free State became the first British Commonwealth country to have a separate or non-British representative in Washington, D.C.. The new state also exchanged diplomats with many other European nations.

The Anglo - Irish Treaty itself also gave the Irish much more freedom than many other dominions. The Oath of Allegiance in Ireland was much less royalist than in Canada or Australia. The king's representative in Ireland was Irish, unlike the other dominions, and although the head of state was the king, power was derived from the Irish people and not him. There were also questions raised about the word "treaty". The British claimed it was an internal affair while the Irish saw it as an international agreement between two independent states, a point which was accepted by the League of Nations, when that body registered the Treaty in 1924.

During the ten years that Cosgrave and Cumann na nGaedheal were in power they adopted a conservative economic policy. Taxation was kept as low as possible and the budget was balanced to avoid borrowing. The Irish currency remained linked to the British currency, resulting in the overvaluation of the Irish pound. Free trade was advocated as opposed to protection, but moderate tariffs were introduced on some items.

The new government decided to concentrate on developing agriculture, while doing little to help the industrial sector. Agriculture responded well with stricter quality control being introduced and the passing of a Land Act to help farmers buy their farms. Also, the Irish Sugar Company and the Agricultural Credit Corporation were established to encourage growth. However, the economic depression that hit in the 1930s soon undid the good work of Cosgrave and his ministers. Industry was seen as secondary to agriculture and little was done to improve it. The loss of the north-east of Ireland had a bad effect on the country as a whole. However, the Electricity Supply Board, with the first national grid in Europe, was established to provide employment and electricity to the new state.

A general election was not necessary until the end of 1932, however, Cosgrave called one for February of that year. There was growing unrest in the country and a fresh mandate was needed for an important Commonwealth meeting in the summer. Another reason for calling the election early was the pending Eucharistic Congress to be held in June. This was a major national and international event. Cosgrave, as a devout Catholic like most of his cabinet, had invested much time in the build-up to it and wished it to procede without any tension fron a pending General election. In the event Eamon de Valera and Fianna Fail were the ones to derive all the kudos from that event. Cumann na nGaedheal fought the election on its record of providing ten years of honest government and political and economic stability. Instead of developing new policies the party played the "red card" by portraying the new party, Fianna Fáil, as communists. Fianna Fáil offered the electorate a fresh and popular manifesto of social reform. Unable to compete with this Cosgrave and his party lost the election, and a minority Fianna Fáil government came to power.

Following the general election Cosgrave assumed the nominal role of Leader of the Opposition. Fianna Fáil were expected to have a short tenure in government, however, this turned out to be a sixteen year period of rule by the new party. In 1933 three groups, Cumann na nGaedheal, the National Centre Party and the National Guard came together to form a new political force, Fine Gael - the United Ireland Party. Cosgrave became the first parliamentary leader of the new party, serving until his retirement in 1944. During that period the new party failed to win a general election. Cosgrave retired as leader of the party and from politics in 1944.

An effective and good chairman rather than a colourful or charismatic leader, he led the new state during the more turbulent period of its history, when the legislation necessary for the foundation of a stable independent Irish polity needed to be pushed through. Cosgrave's governments in particular played a crucial role in the evolution of the British Empire into the British Commonwealth, with fundamental changes to the concept of the role of the Crown, the governor - generalship and the British Government within the Commonwealth.

In overseeing the establishment of the formal institutions of the state his performance as its first political leader may have been undervalued. In an era when democratic governments formed in the aftermath of the First World War were moving away from democracy and towards dictatorships, the Free State under Cosgrave remained unambiguously democratic, a fact shown by his handing over of power to his one-time friend, then rival, Éamon de Valera, when de Valera's Fianna Fáil won the 1932 general election, in the process killing off talk within the Irish Army of staging a coup to keep Cosgrave in power and de Valera out of it.

Perhaps the best endorsement made of Cosgrave came from his old rival, with whom he was reconciled before his death, Éamon de Valera. De Valera once in 1932 and later close to his own death, made two major comments. To an interviewer, when asked what was his biggest mistake, he said without a pause, "not accepting the Treaty". To his own son, Vivion, weeks after taking power in 1932 and reading the files on the actions of Cosgrave's governments in relation to its work in the Commonwealth, he said of Cosgrave and Cosgrave's ministers ". . . when we got in and saw the files. . . they did a magnificent job, Viv. They did a magnificent job."

William T. Cosgrave died on 16 November 1965, aged 85. The Fianna Fáil government under Seán Lemass awarded him the honour of a state funeral, which was attended by the cabinet, the leaders of all the main Irish political parties, and Éamon de Valera, then President of Ireland. He is buried in Goldenbridge Cemetery in Inchicore. Richard Mulcahy said, "It is in terms of the Nation and its needs and its potential that i praise God who gave us in our dangerous days the gentle but steel-like spirit of rectitude, courage and humble self - sacrifice , that was Liam T. Cosgrave". Cosgrave's son, Liam, succeeded his father as a TD in 1944 and went on to become leader of Fine Gael from 1965 to 1977 and Taoiseach from 1973 to 1977. W.T.'s grandson, also called Liam also served as a TD and as Senator and his grand daughter Louise Cosgrave who served as a Dún Laoghaire - Rathdown County Councillor from 1999 to 2008.