The most notorious incident of his tenure as mayor occurred in 1863, when he physically attacked a British commander who boasted of an affair with Cornish's wife.

Cornish often resorted to dubious means to win elections, and received assistance from members of the local Orange Order.

He was defeated for the mayoralty in 1865, when his opponent David Glass successfully petitioned for the local militia to oversee the civic proceedings.

Cornish ran for the Legislative Assembly of Ontario in the 1871 provincial election, but was defeated by John Carling, a local brewer and fellow Conservative.

Lepine had been the adjutant-general in Louis Riel's provisional government, and his arrest sparked bitter divisions among the province's English-speaking and French-speaking communities.

Early in 1875, opposition leader John Norquay entered the cabinet of Premier Davis and brought several of his Anglophone followers to the government side.

He declared himself a "National" in federal politics, and is generally considered to have been a Conservative, albeit of an independent stripe, during his time in the Manitoba legislature.

Like John Christian Schultz, he gradually left his Ontario Conservative background, and sometimes aligned himself with a "Liberal" position when opposing the provincial government.

In his history of Manitoba's legal system, Bruce MacFarlane describes Cornish as "by most accounts a brash and rude man, but extremely intelligent.