Louis St-Laurent was born in Compton, Quebec, a village in the Eastern Townships to Jean-Baptiste-Moïse Saint-Laurent, a French Canadian, and Mary Anne Broderick, an Irish Canadian. He grew up fluently bilingual. His English had a noticeable Irish brogue, while his gestures (such as a hunch of the shoulders) were French.

He received degrees from St. Charles Seminary (B.A. 1902) and Université Laval (LL.L. 1905). He was offered, but declined, a Rhodes Scholarship upon this graduation from Laval in 1905. In 1908 he married Jeanne Renault (1886 – 1966) with whom he had two sons and three daughters. St-Laurent worked as a lawyer from 1905 to 1941, also becoming a professor of law at Université Laval in 1914. St-Laurent practised corporate and constitutional law in Quebec and became one of the country's most respected counsel. He served as President of the Canadian Bar Association from 1930 to 1932.

St-Laurent's father, a Compton shopkeeper, was a staunch supporter of the Liberal Party of Canada and was particularly enamoured with Sir Wilfrid Laurier. When Laurier led the Liberals to victory in the 1896 election, 14 year old Louis relayed the election returns from the telephone in his father's store. However, while an ardent Liberal, Louis remained aloof from active politics for much of his life, focusing instead on his legal career and family. He became one of Quebec's leading lawyers and was so highly regarded that he was offered a position in the Cabinet of the Conservative Prime Minister Arthur Meighen in 1926 and was offered a seat as a justice in the Supreme Court of Canada.

It was not until he was nearly 60 that St-Laurent finally agreed to enter politics when Liberal Prime Minister William Lyon Mackenzie King appealed to his sense of duty in late 1941. Following the death of his Quebec lieutenant, Ernest Lapointe, in November 1941, King was well aware of the need for the government to have a strong, well respected member of cabinet to serve as a new deputy for Quebec to help deal with the volatile conscription issue. King had been in his political infancy when he witnessed the effect that conscription had on the nation during World War I. He had seen Prime Minister Robert Borden polarize the country and marginalize Quebec for standing against conscription, with the effect of seriously jeopardizing national unity - a situation he was determined to avoid. No Quebec or francophone members of Mackenzie's cabinet or government were willing to step into the role, but many recommended St-Laurent, a longtime Liberal supporter, as an ideal candidate. On these recommendations, Mackenzie King recruited St-Laurent to his wartime cabinet as Minister of Justice and appreciating the gravity of the appointment and the situation St-Laurent agreed to go to Ottawa, but only on the understanding that his foray into politics was temporary and that he would return to Quebec at the conclusion of the war.

King appointed St-Laurent as Minister of Justice and Attorney General, Lapointe's old post, on December 9. King felt safe in making this appointment because St-Laurent was slated to run in Lapointe's old riding, Quebec East, in a February 1942 by-election. Both parties had agreed not to contest by-elections for the war's duration, but it is very likely that St-Laurent would have won in any case; at the time Quebec East was one of the safest Liberal ridings in Canada.

St-Laurent supported King's decision to introduce conscription in 1944, despite the lack of support from other French Canadians (Conscription Crisis of 1944). His support prevented more than a handful of Quebec Liberal Members of Parliament (MPs) from leaving the party, and was therefore crucial to keeping the government and the party united.

King came to regard St-Laurent as his most trusted minister and natural successor. He persuaded St-Laurent that it was his duty to remain in government following the war in order to help with the construction of a post war international order and promoted him to the position of Secretary of State for External Affairs in 1945, a portfolio King had previously always kept for himself. In this role, St-Laurent represented Canada at the Dumbarton Oaks Conference and San Francisco Conference that led to the founding of the United Nations (UN). At the conferences, St-Laurent, compelled by his belief that the UN would be ineffective in times of war and armed conflict without some military means to impose its will, advocated the adoption of a UN military force. This force he proposed would be used in situations that called for both tact and might to preserve peace or prevent combat. In 1956, this idea was actualized by St-Laurent and his Secretary of State for External Affairs Lester B. Pearson in the development of UN Peacekeepers that helped to put an end to the Suez Crisis.

In 1948, King retired, and quietly persuaded his senior ministers to support St-Laurent's selection as the new Liberal leader at the Liberal leadership convention of August 1948. St-Laurent won, and was sworn in as Prime Minister of Canada on November 15.

The Canadian economy was one of the strongest in the world in the period immediately following the end of the war. The prosperity lasted for more than a decade, significantly expanding the Canadian national infrastructure.

In the 1949 federal election that followed his ascension to the Liberal leadership many wondered, including Liberal party insiders, if St-Laurent would appeal to the post-war populace of Canada. On the campaign trail, St-Laurent's image was developed into somewhat of a 'character' and what is considered to be the first 'media image' to be used in Canadian politics. St-Laurent chatted with children, gave speeches in his shirt sleeves, and had a 'common touch' that turned out to be appealing to voters. At one event during the 1949 election campaign, he disembarked his train and instead of approaching the assembled crowd of adults and reporters, gravitated to, and began chatting with, a group of children on the platform. A reporter submitted an article entitled "'Uncle Louis' can't lose!" which earned him the nickname "Uncle Louis" in the media (Papa Louis in Quebec). With this common touch and broad appeal, he subsequently led the party to victory in the election against the Progressive Conservative Party led by George Drew. The Liberals won 190 seats — the most in Canadian history at the time, and still a record for the party.

His reputation as Prime Minister was impressive. He demanded hard work of all of his MPs and Ministers, and worked hard himself. He was reputed to be as knowledgeable on some ministerial portfolios as the ministers responsible themselves. To that end, Jack Pickersgill (a minister in St-Laurent's cabinet) said as prime minister St-Laurent had: "as fine an intelligence as was ever applied to the problems of government in Canada. He left it a richer, a more generous and more united country than it had been before he became prime minister."

St-Laurent led the Liberals to another powerful majority in the 1953 federal election. He lost several seats, but still dominated the Canadian House of Commons.

St-Laurent and his cabinet oversaw Canada's expanding international role in the postwar world. His stated desire was for Canada to occupy a social, military and economic 'Middle power' role in the post World War II world.

Militarily, St-Laurent was a leading proponent of the establishment of the North Atlantic Treaty Organization (NATO) in 1949, serving as an architect and signatory of the treaty document. Involvement in such an organization marked a departure from King who had been reticent about joining a military alliance. Under his leadership, Canada supported the United Nations (U.N.) in the Korean War and committed the third largest overall contribution of troops, ships and aircraft to the U.N. forces to the conflict. Troops to Korea were selected on a voluntary basis. In 1956, under his direction, St-Laurent's Secretary of State for External Affairs Lester B. Pearson, helped solve the Suez Crisis in 1956 between Great Britain, France, Israel and Egypt, bringing forward St-Laurent's 1946 views on a U.N. military force in the form of the United Nations Emergency Force (UNEF) or peacekeeping. It is widely believed that the activities directed by St-Laurent and Pearson could well have avoided a nuclear war. These actions were recognized when Pearson won the 1957 Nobel Peace Prize.

St-Laurent was an early supporter of British Prime Minister Clement Attlee's proposal to transform the British Commonwealth from a club of white dominions into a multi-racial partnership. The leaders of the other "white dominions" were less than enthusiastic. It was St-Laurent who drafted the London Declaration, recognizing King George VI as Head of the Commonwealth as a means of allowing India to remain in the international association once it became a republic.

St-Laurent's government was modestly progressive and fiscally conservative, taking taxation surpluses no longer needed by the wartime military and paying back in full Canada's debts accrued during the First World War, the Great Depression and World War II. With remaining revenues, St-Laurent oversaw the expansion of Canada's social programs, including establishment of the Canada Council to support the arts, and the gradual expansion of social welfare programs such as family allowances, old age pensions, government funding of university and post-secondary education and an early form of Medicare termed Hospital Insurance at the time, that lay the groundwork for Tommy Douglas' healthcare system in Saskatchewan and Pearson's nationwide universal healthcare in the late 1960s. In addition, he modernized and established new social and industrial policies for the country during his time in the prime minister's office.

In 1949, the former lawyer of many Supreme Court cases, St-Laurent ended the practice of appealing Canadian legal cases to the Judicial Committee of the Privy Council of Great Britain, making the Supreme Court of Canada the highest avenue of legal appeal available to Canadians. In that same year, St-Laurent negotiated the British North America (No. 2) Act, 1949, with Britain which 'partially patriated' the Canadian Constitution, most significantly giving the Canadian Parliament the authority to amend portions of the constitution. Also in 1949, following two referenda within the province, St-Laurent and Premier Joey Smallwood negotiated the entry of Newfoundland into Confederation.

In 1952, he advised Queen Elizabeth II to appoint Vincent Massey as the first Canadian born Governor-General. Each of the aforementioned actions were and are seen as significant in furthering the cause of Canadian autonomy from Britain and developing a national identity on the international stage.

In 1956, using the Constitutional taxation authority of the federal level of government, St-Laurent's government introduced the policy of "Equalization payments" which redistributes taxation revenues between provinces to assist the poorer provinces in delivering government programs and services, a move that has been considered a strong one in solidifying the Canadian federation, particularly with his home province of Québec. The government also engaged in massive public works and infrastructure projects such as building the Trans-Canada Highway (1949), the St. Lawrence Seaway (1954) and the Trans-Canada Pipeline. It was this last project that was to sow the seeds that led to the downfall of the St-Laurent government.

St-Laurent was initially very well-received by the Canadian public, but by 1957, "Uncle Louis" and his government began to appear tired, old and out of touch. The government was also perceived to have grown too close to business interests. The 1956 Pipeline Debate led to the widespread impression that the Liberals had grown arrogant in power when the government invoked closure on numerous occasions in order to curtail debate and ensure that its Pipeline Bill passed by a specific deadline. St. Laurent was criticized for a lack of restraint exercised on his minister C.D. Howe, who was widely perceived as extremely arrogant. Western Canadians felt particularly alienated by the government, believing that the Liberals were kowtowing to interests in Ontario and Quebec and the United States. (The opposition accused the government of accepting overly costly contracts that could never be completed on schedule - in the end the pipeline was completed early and under budget). The pipeline conflict turned out to be meaningless, insofar as the construction work was concerned, since pipe could not be obtained in 1956 from a striking American factory, and no work could have been done that year. But the ensuing uproar in Parliament had a lasting impression on the electorate, and was a decisive factor in the Liberal government's defeat at the hands of the Progressive Conservative Party led by John Diefenbaker in the 1957 election. Because the Liberals were still mostly classically liberal, Diefenbaker promised to outspend the incumbent Liberals, who campaigned on plans to stay the course of fiscal conservatism they had followed through St-Laurent's term in the 1940s and 1950s.

St-Laurent was the first Prime Minister to live in the present official residence of the Prime Minister of Canada: 24 Sussex Drive, from 1951 to the end of his term in office.

The defeat in the 1957 was marked by controversy within the Liberal party and the Parliament. The Liberals had actually won more popular support (actual votes cast) than the Progressive Conservatives (40.75% Liberals to 38.81% PC), but the Conservatives took the greatest number of seats with 112 PC candidates elected to serve out of the House of Commons 265 seats (42% of the House). The Liberals took 104 seats (39.2%). Some ministers wanted St-Laurent to stay on and offer to form a minority government, following the logic that the popular vote had supported them and even though their Parliamentary minority was smaller than the Conservatives, the Liberals' more recent governmental experience would make them a more effective minority.

Another option circulated within the party saw the balance of power to be held by either the Co-operative Commonwealth Federation (CCF) and their 25 seats or Social Credit Party of Canada with their 15. St-Laurent was encouraged by others to reach out to the CCF and at least four of six independent/small party MPs to form a coalition majority government, which would have held 134 of the 265 or 50.1% of the seats in Parliament. St-Laurent, however, decided that the nation had passed a verdict against his government and his party and he resigned as Prime Minister rather than be seen as clinging to office.

After a short period as Leader of the Opposition and now more than 75 years old, St- Laurent's motivation to be involved in politics was gone. He announced his intention to retire from politics. St-Laurent was succeeded as Liberal Party leader by his former Secretary of State for External Affairs and representative at the United Nations, Lester B. Pearson, at the party's leadership convention in 1958. After his political retirement, he returned to practising law and living quietly and privately with his family. During his retirement, he was called into the public spotlight one final time in 1967 for the inception of the award, he was made a Companion of the Order of Canada.

Louis Stephen St-Laurent died from natural causes on July 25, 1973, in Quebec City, Quebec, aged 91, and was laid to rest at St. Thomas Aquinas Cemetery in his hometown of Compton, Quebec. He is survived by granddaughter Louise Mignault and grandson Louis St-Laurent II.

St. Laurent was ranked #4 on a survey of the first 20 prime ministers (through Jean Chrétien) of Canada done by Canadian historians, and used by J.L. Granatstein and Norman Hillmer in their book Prime Ministers: Ranking Canada's Leaders.