Thomas Mitchell was born in Philadelphia on April 17, 1845, the son of John Cowell Mitchell (Class of 1835) and Rebecca Nicholas.
Mitchell entered the University in 1861 as member of the Class of 1865. He was a member of the Phi Kappa Sigma fraternity and vice president of the Glee Club.
During the Civil War, Mitchell was a first lieutenant in the University Light Artillery and also commissioned as a first lieutenant in the 198th Pennsylvania Regiment of the United States Volunteers. He was captain of the University Light Artillery in his senior year at the time he left the University for military service. In May of 1865, Mitchell served on the staff of General Joshua L. Chamberlain. Before being mustered out on June 4, 1865, he was acting assistant adjutant general for the brigade at the surrender of Robert E. Lee. He became a lawyer and then, in 1879 and 1880, a judge of the First Judicial District of Colorado.
Thomas Mitchell married Lucy Breck Reed. He died in 1909 in Mayfield, California.