Three years later he was appointed District Attorney of Orleans County, and after the New York State Constitution of 1846 became effective, was elected by the people to the same office, serving until the end of 1850.

Seymour was defeated by Washington Hunt, the Whig candidate, by about two hundred majority, but Church ran ahead of his ticket and was elected.

During the Civil War, Church "vilified the Lincoln administration for seeking to 'absorb, centralize and consolidate the rights and powers of the loyal States in the general government.

'"[3] A delegate to the 1844, 1860, 1864 and 1868 Democratic National Conventions, Church, in 1860, advocated the nomination of Stephen A. Douglas.

At the Democratic National Convention, held in New York City in July 1868, he was named by the delegation from New York State as its choice for the nomination for the presidency, and his name was presented to the convention by Samuel J. Tilden, the chairman of the delegation, who cast the vote of the State for him for the first seven ballots.

New York then switched to Thomas A. Hendricks from the eighth to the twenty-second ballots until a break was made by other states to Horatio Seymour, the chairman of the convention, who was then nominated.

In the spring of 1870, Church was nominated by the Democratic convention for chief judge of the Court of Appeals.