John Cochrane was born August 27, 1813, in Palatine, Montgomery County, New York. He studied at Union College and then transferred to Hamilton College, from which he graduated in 1831. Afterwards he studied law, was admitted to the bar in 1834, practiced in Oswego and Schenectady, and then moved to New York City.
He was elected as a Democratic to the 35th and 36th United States Congresses and served from 1857 to 1861, where he focused on land reform, revenue, and other public questions. After the attack on Fort Sumter, Cochrane became a colonel of the 65th New York Volunteer Infantry (1st United States Chasseurs), which he commanded in the Peninsular Campaign. In July 1862, he was promoted to brigadier general of volunteers. He resigned his commission in 1863.
John Cochrane was New York State Attorney General during the years 1864 to 1865. While in office, he discovered that the waters of Kills and Raritan Bay were considered part of the Hudson River in colonial times. This discovery shows Staten Island east of the waters of the Hudson, which is the easterly charter boundary of New Jersey, and substantiates New York's claim to the island which had been disputed by New Jersey.
In 1872, John Cochrane became a member of the Common Council of New York City, and as its President became acting Mayor during the temporary retirement of Mayor Hall following the Tweed Ring disclosures.
John Cochrane died February 7, 1898 in Manhattan, New York and was buried in the Civil War section of Albany Rural Cemetery.
Death List of a Day. New York Times, February 9, 1898
Charles Morris. Men of the Century, an Historical Work (1896) States at War, Volume 2: A Reference Guide for New York in the Civil War, Richard F. Miller, ed. (2014)