His father, William Warren Baldwin (d. 1844), went to Canada from Ireland in 1798; though a man of wealth and good family and a devoted member of the Church of England, he opposed the religious and political oligarchy which was then at the head of Canadian affairs, and brought up his son in the same principles. Robert Baldwin was called to the Bar in 1825, and entered into partnership with his father. In 1829 he was elected a member of the parliament of Upper Canada for the town of York, but was defeated in the following year and retired for a time into private life. During the next six years, he so constantly advocated a responsible executive as the one cure for the political and economic evils of the time that he was known as "the man of one idea." In 1836 he was called by Sir Francis Bond Head (1793 – 1875), the lieutenant-governor, to the executive council, hut finding himself without influence, and compelled to countenance measures to which he was opposed, he resigned within a month. Though a reformer, he strongly disapproved of the rebellion of 1837 – 1838.
On the union of the two Canadas he became (1841) a member of the executive council under Lord Sydenham, but soon resigned on the question of responsible government. In 1842 he formed the first Liberal administration, in connection with Mr (afterwards Sir) L. H. Lafontaine, but resigned the next year, after a quarrel with the governor-general, Sir Charles Metcalfe, on a question of patronage, in which he felt that of responsible government to be involved. At the general election which followed, the governor-general was sustained by a narrow majority, but in 1847 the Liberals were again returned to power, and he and Mr Lafontaine formed their second administration under Lord Elgin and carried numerous important reforms, including the freeing from sectarian control of the University of Toronto and the introduction into Upper Canada of an important municipal system.
Internal dissensions soon began to appear in the Liberal party, and in 1851 Mr Baldwin resigned. The special struggle leading to his resignation was an attempt to abolish the court of chancery of Upper Canada, whose constitution was due to a measure introduced by Baldwin in 1849. The attempt, though defeated, had been supported by a majority of the representatives from Upper Canada, and Baldwin's fastidious conscience took it as a vote of want confidence. A deeper reason was his inability to approve of the advanced views of the Radicals, or "Clear Grits," as they came to be called. On seeking re-election in York, he declined to give any pledge on the burning question of the Clergy Reserves and was defeated. In 1853 the Liberal-Conservative party, formed in 1854 by a coalition, attempted to bring him out as a candidate for the upper house, which was at this date elective, but though he had broken with the advanced reformers, he could not approve of the tactics of their opponents, and refused to stand. He died on the 9th of December 1858. Even those who most bitterly attacked his measures admitted the purity and unselfishness of his motives. After the concession of responsible government, he devoted himself to bringing about a good understanding between the English and French-speaking inhabitants of Canada, and his memory is held as dear among the French Canadians as in his native province of Ontario.