During the American Revolution, Cannon was one of the leaders of a radical faction campaigning for independence, in opposition to Philadelphia's majority support for an accommodation with Britain.
According to Christopher Marshall's diary, Cannon was a leading organizer in private meetings held to select radical candidates for the 1776 elections to the Pennsylvania Provincial Assembly.
The moderate faction won a majority in the elections on May 1, 1776, but were outmaneuvered by the radicals, who successfully persuaded the Continental Congress to resolve that all local governments deriving their authority from the British Crown should be "totally suppressed."
The Convention made him a Justice of the Peace for Pennsylvania, and also enabled him to serve as a member of the Council of Safety from July 24, 1776, to December 4, 1777.
Drawing on the language of the Declaration of Independence, the new constitution conferred detailed rights on citizens and enfranchised all tax-paying free men, who would vote in annual elections for a unicameral legislature.