In 1816, Coolidge was commissioned as a captain in the Vermont Militia, and assigned to 1st Regiment, 4th Division.

[6] Coolidge was one of Vermont's presidential electors in 1844, and cast his ballot for Henry Clay and Theodore Frelinghuysen.

He served as President of the Vermont Whig Convention in 1847, which passed resolutions opposing the Mexican–American War and the acquisition of territory by conquest, and in favor of the Wilmot Proviso.

Coolidge's anti-slavery views also included the idea of returning freed slaves to Africa as settlers, and he was active in both the American Colonization Society and the Vermont Colonization Society.

He became a Republican when the party was founded in the 1850s, and served in the Vermont State Senate from 1853 to 1857,[8] after which he again returned to his law practice.