The son of Count Károly Andrássy and Etelka Szapáry, he was born in Vlachovo (now in Rožňava District, Slovakia) in the then Kingdom of Hungary. The son of a liberal father who belonged to the political opposition, at a time when to oppose the government was very dangerous, Andrássy at a very early age threw himself into the political struggles of the day, adopting at the outset the patriotic side.

Count István Széchenyi was the first adequately to appreciate his capacity, when in 1845 the young man first began his public career as president of the society for the regulation of the waters of the Upper Tisza river.

In 1846, he attracted attention by his bitter articles against the government in Lajos Kossuth's paper, the Pesti Hírlap, and was returned as one of the Radical candidates to the diet of 1848, where his generous, impulsive nature made him one of the most thorough going of the patriots.

When the Croats under Josip Jelačić attempted to return Međimurje, which was then part of Hungary, to Croatia, Andrássy placed himself at the head of the gentry of his county, and served with distinction at the battles of Pákozd and Schwechat, as Arthur Görgey's adjutant (1848). Towards the end of the war Andrássy was sent to Constantinople by the revolutionary government to obtain at least the neutrality of Ottoman Empire during the struggle.

After the catastrophe of Világos he migrated first to London and then to Paris. On 21 September 1851 he was hanged in effigy by the Austrian government for his share in the Hungarian revolt. He employed his ten years of exile in studying politics in what was then the centre of European diplomacy, and it is memorable that his keen eye detected the inherent weakness of the second French empire beneath its imposing exterior.

Andrássy returned home from exile in 1858, but his position was very difficult. He had never petitioned for an amnesty, steadily rejected all the overtures both of the Austrian government and of the Magyar Conservatives (who would have accepted something short of full autonomy), and clung enthusiastically to Ferenc Deák's party.

On 21 December 1865 he was chosen vice-president of the diet, and in March 1866 became president of the subcommittee appointed by the parliamentary commission to draw up the Composition (commonly known as the Austro-Hungarian Compromise of 1867) between Austria and Hungary, of which the central idea, that of the "Delegations," originated with him. It was said at that time that he was the only member of the commission who could persuade the court of the justice of the national claims.

After the Battle of Königgrätz he was formally consulted by Emperor Franz Joseph for the first time. He advised the reestablishment of the constitution and the appointment of a responsible ministry. On 17 February 1867 the king appointed him the first constitutional Hungarian premier. It was on this occasion that Ferenc Deák called him "the providential statesman given to Hungary by the grace of God."

As premier, Andrássy by his firmness, amiability and dexterity as a debater, soon won for himself a commanding position. Yet his position continued to be difficult, inasmuch as the authority of Deák dwarfed that of all the party leaders, however eminent. Andrássy chose for himself the departments of war and foreign affairs. It was he who reorganized the Honvéd system (state army), and he used often to say that the regulation of the military border districts was the most difficult labour of his life.

On the outbreak of the Franco-Prussian War of 1870, Andrássy resolutely defended the neutrality of the Austrian monarchy, and in his speech on 28 July 1870 warmly protested against the assumption that it was in the interests of Austria to seek to recover the position she had held in Germany before 1863. On the fall of Beust (6 November 1871), Andrássy stepped into his place. His tenure of the chancellorship was epoch-making.

Hitherto the empire of the Habsburgs had never been able to dissociate itself from its Holy Roman traditions. But its loss of influence in Italy and Germany, and the consequent formation of the Dual State, had at length indicated the proper, and, indeed, the only field for its diplomacy in the future – the Near East, where the process of the crystallization of the Balkan peoples into nationalities was still incomplete. The question was whether these nationalities were to be allowed to become independent or were only to exchange the tyranny of the sultan for the tyranny of the tsar. Hitherto Austria had been content either to keep out the Russians or share the booty with them. She was now, moreover, in consequence of her misfortunes deprived of most of her influence in the councils of Europe.

It was Andrassy who recovered for her proper place in the European concert. First he approached the German emperor; then more friendly relations were established with the courts of Italy and Russia by means of conferences at Berlin, Vienna, St Petersburg and Venice. The recovered influence of Austria was evident in the negotiations which followed the outbreak of serious disturbances in Bosnia in 1875. The three courts of Vienna, Berlin and St Petersburg had come to an understanding as to their attitude in the Eastern question, and their views were embodied in the dispatch, known as the "Andrássy Note", sent on 30 December 1875 by Andrássy to Count Beust, the Austrian ambassador to the Court of St James. In it he pointed out that the efforts of the powers to localize the revolt seemed in danger of failure, that the rebels were still holding their own, and that the Ottoman promises of reform, embodied in various firmans, were no more than vague statements of principle which had never had, and were probably not intended to have, any local application. In order to avert the risk of a general conflagration, therefore, he urged that the time had come for concerted action of the powers for the purpose of pressing the Porte to fulfil its promises. A sketch of the more essential reforms followed: the recognition rather than the toleration of the Christian religion; the abolition of the system of farming the taxes; and, in Bosnia and Herzegovina, where the religious was complicated by an agrarian question, the conversion of the Christian peasants into free proprietors, to rescue them from their double subjection to the Muslim Ottoman landowners. In Bosnia and Herzegovina elected provincial councils were to be established, life-term judges appointed and individual liberties guaranteed. Finally, a mixed commission of Muslims and Christians was to be empowered to watch over the carrying out of these reforms.

The fact that the sultan would be responsible to Europe for the realization of his promises would serve to allay the natural suspicions of the insurgents. To this plan both Britain and France gave a general assent, and the Andrássy Note was adopted as the basis of negotiations. When war became inevitable between Russia and the Porte, Andrássy arranged with the Russian court that, in case Russia prevailed, the status quo should not be changed to the detriment of the Austrian monarchy. When, however, the Treaty of San Stefano threatened a Russian hegemony in the Near East, Andrássy concurred with the German and British courts that the final adjustment of matters must be submitted to a European congress.

At the Berlin Congress in 1878 he was the principal Austrian plenipotentiary, and directed his efforts to diminish the gains of Russia and aggrandize the Dual Monarchy. The latter object was gained by the occupation of Bosnia-Herzegovina under a mandate from the congress. This occupation was most unpopular in Hungary, both for financial reasons and because of the strong philo-Turk sentiments of the Magyars, but the result brilliantly justified Andrássy's policy. Nevertheless he felt constrained to bow before the storm, and placed his resignation in the emperor's hands (October 8, 1879). The day before his retirement he signed the offensive-defensive alliance with Germany, which placed the foreign relations of Austria-Hungary once more on a stable footing.

After his retirement, Andrássy continued to take an active part in public affairs both in the Delegations and in the Upper House. In 1885 he warmly supported the project for the reform of the House of Magnates, but on the other hand he jealously defended the inviolability of the Composition of 1867, and on 5 March 1889 in his place in the Upper House spoke against any particularist tampering with the common army. In the last years of his life he regained his popularity, and his death on 18 February 1890, aged 66, was mourned as a national calamity. There is a plaque dedicated to him in the town of Volosko where he died (between Rijeka and Opatija in present-day Croatia). It is located just above the restaurant Amfora.

He was the first Magyar statesman who, for centuries, had occupied a European position. It has been said that he united in himself the Magyar magnate with the modern gentleman. His motto was: "It is hard to promise, but it is easy to perform." If Deak was the architect, Andrássy certainly was the master-builder of the modern Hungarian state.

By his wife, the countess Katinka Kendeffy, whom he married in Paris in 1856, Count Andrássy left two sons, and one daughter, Ilona (b. 1859). Both the sons gained distinction in Hungarian politics. The eldest, Tivadar András (Theodore Andreas) (born 10 July 1857), was elected vice-president of the Lower House of the Hungarian parliament in 1890. The younger, Gyula (born 30 June 1860), also had a successful political career.

According to a very common legend, Count Andrássy had a long lasting romance with Queen Elisabeth (Sissy), wife of Emperor and King Franz-Josef of Austria-Hungary, and fathered their only son, Archduke Rudolf, although there is no evidence for this story, except for the strong sympathy and devotion of both Sissy and Rudolf towards Hungary, its culture and national customs (they were both fluent in Hungarian and regarded Hungarian poetry highly). His great, great, great granddaughter Dame Laura Everett and great, great, great grandson Richard Everett are currently living in Scissett, Huddersfield. She is a highly regarded member of society who is fluent in Hungarian.