In 1828, he was elected to the Legislative Assembly of Upper Canada for Halton as a Reformer, and was considered radical enough to be endorsed by the Colonial Advocate. He did not run in 1830 but was reelected in 1834. Despite his radical views, he was not implicated in the 1837 Rebellion, thus enabling him to become one of the major Reform leaders. He was elected to the 1st Parliament of the Province of Canada in East Halton, and immediately associated with the "ultra-Reform" wing headed by Robert Baldwin; however, he soon gained a reputation for being a within the party. In 1841, he supported the District Councils Act, which was opposed by the other Reformers. In stating his support, he declared, "I came here to build up a countrynot a party. I shall vote for the bill." In 1843, he sided with the Tories in opposing the move of the capital from Kingston to Montreal, and also expressed his disapproval when the Reform government resigned in protest over a dispute with the Governor, Charles Metcalfe. As a result, he was ostracized within the party, and later rejected for nomination in 1844 in favour of John Wetenhall. He decided to run as well, thus splitting the vote and allowing George Chalmers, the Conservative candidate, to win. Upon the rise of the Clear Grits, Hopkins joined them and became one of the revered "old Reformers". When Malcolm Cameron resigned from the Baldwin-Lafontaine government to join them, he was replaced in the Cabinet by Wetenhall, who was obliged to run in a mandatory byelection in 1850. Hopkins, supported by Cameron, chose to run against him, and was the winner in what turned out to be a very vicious campaign. As one of the five Clear Grits in the Legislative Assembly, he was effective in opposition, and helped to bring down the Baldwin-Lafontaine ministry in 1851. Hopkins chose not to run in the resulting election and retired from politics, later moving to Hamilton in 1870. Hopkins came to be known as "a grand old man of Ontario Liberalism." He died in Toronto in 1880 at the home of his son-in-law William Leggo. At his death, he was the oldest parliamentarian in all of Canada, and his pallbearers included Alexander Mackenzie and Oliver Mowat. He was buried at the Toronto Necropolis.
