Afterwards he studied law, was admitted to the bar in 1834, practiced in Oswego and Schenectady, and then moved to New York City.

At the outbreak of the American Civil War he became a colonel of the 65th New York Volunteer Infantry (also known as the 1st United States Chasseurs), which he commanded in the Peninsular Campaign.

In July 1862, he was promoted to brigadier general of volunteers, but resigned his commission in February 1863, ostensibly on account of failing health but more likely as a result of his political maneuverings after Fredericksburg.

As leader of the New York delegation to the Liberal Republican Convention at Cincinnati in 1872, he was instrumental in securing the nomination of Horace Greeley for the presidency.

Cochrane, who was a member of the Society of the Cincinnati, died at his home at 7 East Sixty-Second Street in Manhattan, and was buried at Albany Rural Cemetery in Menands, New York.