After the war Tannehill settled in Pittsburgh, as did a number of other Revolutionary War officers, including his brother Lt. Josiah Tannehill. He initially engaged in agricultural pursuits and was a tavern owner and vintner, president of the Pittsburgh Fire Co., and a trustee of the first Presbyterian church in Pittsburgh. He later served as a local Justice of the Peace; lieutenant colonel of Westmoreland Co. militia starting in 1788; an original member of the Board of Directors of the Pittsburgh branch of the Bank of Pennsylvania, the first bank in Pittsburgh, starting in 1804; one of five turnpike commissioners for the state starting in 1811; major general of Pennsylvania Volunteers during the War of 1812; and president of the Pittsburgh branch of the Bank of the United States starting in 1817.