prime minister of Serbia (1891–92, 1904–05, 1906–08, 1909–11, 1912–18) and prime minister of the Kingdom of Serbs, Croats, and Slovenes (Serbs, Croats, and Slovenes, Kingdom of) (1918, 1921–24, 1924–26). He was one of the founders, in 1918, of the kingdom that would later (from 1929 to 2003) be called Yugoslavia.

Pašić, who was born into a family of modest means, studied engineering in Belgrade and then graduated from the Zürich Polytechnikum, where his interest in contemporary liberalism and democratic institutions was stimulated by the Russian anarchist Mikhail Bakunin (Bakunin, Mikhail Aleksandrovich). Returning to Serbia (1873), he joined the Socialist group led by Svetozar Marković (Marković, Svetozar) and, as editor of the newspaper Oslobodjenje (“Liberation”), became an important exponent of Marković's views. Having concluded that King Milan Obrenović's (Milan IV (or II)) oligarchy (Obrenović dynasty) was depriving Serbia both of progressive leadership and of national perspective, Pašić decided to enter politics actively. Elected to parliament in 1878, he worked, as leader of the opposition, against the authoritarian monarchy in an endeavour to establish a parliamentary democracy. He also helped to found the Radical Party (1881).

When a popular rising instigated against Milan's government by the Radicals in the Timok region (1883) led to further repression and to the severe punishment of many Radical leaders, Pašić was forced to flee through Austria to Bulgaria. After Milan's abdication in favour of his son Alexander (1889), Pašić returned to Serbia from exile and was then elected president of the Skupština (Parliament) and, on two occasions, mayor of Belgrade. Pašić served as premier for the first time from February 1891 to August 1892 and as foreign minister accompanied the new king, Alexander Obrenović (Alexander), on a state visit to Russia (1892), where Pašić established firm personal and political ties with the tsarist regime. He became Serbian minister to St. Petersburg in 1893 but resigned in protest at former king Milan's illegal return to Serbia (1894).

After an unsuccessful attempt on Milan's life in 1899, trumped-up charges of regicide were brought against the members of the Radical Party. Pašić, who was among those sentenced to death, won himself an amnesty and then left the country voluntarily, to return only when Milan had finally withdrawn.

When Alexander was overthrown and the Karadjordjević dynasty, in the person of King Peter I, was restored by the bloody coup d'état of 1903, Pašić finally emerged as the dominant political figure in Serbia. As leader of the Radical Party, he concentrated his efforts on establishing the party both as the backbone of the new regime and as the moving force in Serbian politics. From December 1904 to May 1905 he served as premier and as minister for foreign affairs—displaying great skill by counteracting Austria-Hungary's attempts to impose a tariff war on Serbia. He held both posts again from May 1906 to June 1908 and was again reappointed premier in October 1909, only to be replaced in 1911 by Milovan Milovanović (Milovanović, Milovan), his greatest political rival. Though Pašić cooperated with Milovanović in concluding a pact with Bulgaria—from which was eventually to develop the Balkan League, whose aim was war against Turkey (Ottoman Empire)—younger politicians and many military leaders continually conspired to remove him from his position as party leader, and in 1912 his imminent dismissal was avoided only by Milovanović's sudden death. Thenceforth reinstated as premier and minister for foreign affairs, Pašić led Serbia through two victorious wars (Balkan Wars), the first against Turkey (1912) and the second against Bulgaria (1913).

Despite his increased prestige, further attempts were made to oust Pašić from office in the months preceding the start of World War I. The accession as regent of Prince Alexander (Alexander I) (King Peter's younger son) on June 24, 1914, gave Pašić some support, however, and his position was further confirmed when the threat of war with Austria-Hungary prevented forthcoming new elections from being held.

After the murder of Archduke Francis Ferdinand (Francis Ferdinand, archduke of Austria-Este) by Serb nationalist Gavrilo Princip at Sarajevo on June 28, 1914, Pašić was most compliant in dealing with the formidable terms of Austria-Hungary's ultimatum to Serbia but was nevertheless unable to avert the Austrian declaration of war on Serbia on July 28. World War I initially silenced Serbian political discord: parliament, which had already been dissolved, reassembled at Niš, and in November 1914 a coalition government under Pašić's premiership was formed.

The Austro-German conquest of Serbia forced Pašić's government and the army to withdraw from Serbia to Corfu (winter 1915). Pašić remained prime minister of the government-in-exile throughout World War I. He also remained interested in the union of Serbia with provinces inhabited predominantly by Serbs, but not in the formation of a federal Yugoslavia with Serbs, Croats, and Slovenes autonomous and equal. When the coalition government broke up in 1917, Pašić continued to govern with a homogeneous Radical Cabinet. When his position was still further weakened by the fall of Russia's tsarist regime (1917), he was obliged temporarily to abandon his strict Pan-Serbian attitude and to negotiate on equal terms with Ante Trumbić (Trumbić, Ante)'s Yugoslav Committee, a body of South Slav exiles from Austria-Hungary with its seats in London and Paris. The resulting Corfu Declaration (July 20, 1917) laid down the broad lines for a postwar unified Yugoslav state.

As World War I neared its end, Pašić stubbornly insisted that Serbia, as the dominant political and military force among the South Slavs, had the exclusive right to speak on the Allied side on their behalf. In November 1918, however, Pašić, under pressure from the Serbian opposition and from the Allied governments, joined delegates from the Yugoslav Committee, from the National Council recently formed in Zagreb and from the Serbian opposition, in signing a declaration that provisionally envisaged a Yugoslavia in which the Serbian government would share power with the representatives of Austria-Hungary's former South Slav subjects. But the Serbian government, which Pašić himself had secretly dissuaded, rejected the declaration. As a result, when Austria-Hungary collapsed, the Allies were unable to agree on a solution for the relations of the South Slavs with Serbia, while Italy reasserted its territorial claims to South Slav territory under a secret wartime pact (Treaty of London) made between it and the Allied Powers. Despite the danger, Pašić persevered in his obstructive tactics toward the Yugoslav Committee and the National Council in Zagreb.

Nevertheless, an uneasy compromise was finally achieved when Serbia and the South Slav provinces were united on December 1, 1918, as the Kingdom of Serbs, Croats, and Slovenes (Serbs, Croats, and Slovenes, Kingdom of). Though he was denied the premiership of the kingdom, Pašić went with Trumbić and Vesnić as one of the new state's delegates to the Peace Conference at Versailles (1919).

Pašić failed to comprehend fully the fateful difference between Serbia's homogeneity and the complexity of the new kingdom, which comprised several nations, each with its own distinct historical development and cultural identity. Ignoring requests for individual recognition from Croats, Slovenes, Macedonians, and Bosnian Muslims, he continued to regard them simply as Serbs—albeit Serbs of three religions and several names. When, therefore, he was reappointed premier in 1921 he immediately pushed through parliament a unitary constitution for the new nation that, under the guise of establishing a homogeneous state, actually confirmed the existing Serbian hegemony and, by abolishing historic and autonomous provinces, established a strongly centralized regime under a powerful monarchy. He eliminated the Democrats from the government (winter 1921) and formed an entirely Radical Cabinet. He failed to secure a majority in the elections of March 1923 but stayed safely in office, thanks to blunders by the opposition. Though from July to October 1924 he had to give way to a coalition government under Ljubomir Davidović (Davidović, Ljubomir), by adroit interparty maneuvering he was able immediately afterward to return to power much stronger than before. His relations with King Alexander and with the Anticentralist Croats and Slovenes nevertheless became increasingly strained. In February 1925 Pašić was forced to dissolve parliament, but by adopting drastic measures—among them the imprisonment of Stjepan Radić (Radić, Stjepan) and other Croatian Peasant Party leaders—he secured a small working majority. A temporary political collaboration with Radić later the same year failed to produce a stable government, and, when Radić publicly criticized the still-increasing tendency toward centralization and unification, Pašić had to resign in March 1926. A man of strict honesty in his public life, he was deeply wounded by the implication of his son in a corruption scandal, and he died in December 1926, shortly before his 81st birthday.