In 1875, he was elected to the Ontario legislature as a Conservative, defeating independent candidate L. Montreuil by 1209 votes to 755 in the riding of Essex North.

Patterson became a backbench supporter of Prime Minister John A. Macdonald's government and was re-elected in the federal riding of Essex North in 1882 and 1887.

In spite of this, he was subsequently called into the cabinet of John Abbott, who became prime minister following Macdonald's death following the 1891 election.

He was sworn in as Secretary of State of Canada on January 25, 1892, and was re-elected to parliament on February 22 (defeating Liberal Malcolm Colin Cameron by 25 votes in a Huron West by-election).

He sought to bolster Canada's defensive capabilities and oversaw the construction of fortifications in Esquimalt, British Columbia.

Patterson was responsible for overseeing the end of most state funding for Catholic and francophone schools, although it is not clear that he played a significant role in the matter.