Thomas Tanner was born on April 13, 1843, in Mercer County, Pennsylvania, to Richard Tanner and Catherine Hurley. He grew up and attended school in Mercer County, and his father probably died around 1850. He began studying law in 1861, but he abandoned his studies to join the Union army.
He enlisted on September 1, 1862, and he mustered in as a private in Company A of the 139th Pennsylvania Infantry later that day. He was promoted to corporal on January 1, 1863, and to sergeant on July 1, 1864. The regiment took part in the Battle of Antietam, the Battle of Fredericksburg, the Battle of Gettysburg, and the Overland Campaign. He remained devoted to the Union cause, observing in May 1863 that “I don’t think that I would be contented to stay at home while this war lasts.” He was wounded in the Battle of Cold Harbor on June 2, 1864, and he mustered out on May 19, 1865.
He settled in Sharon, Pennsylvania, around 1870 and earned a living as a lawyer. He married Elma Adams in the 1870s, and their son Adams was born around 1877. He supported the Republican Party and served as president of the local Board of Education. He applied for a federal pension in May 1898 and eventually secured one. He died in Sharon on June 29, 1898.