Thomas Walker was born into slavery around 1847 in Charlottesville, Virginia. By the early 1860s, his owner was Martha M. Schaaff, who lived in Prince George's County, Maryland. Walker enlisted in the Union army on June 25, 1863, at Mason's Island, Virginia, at the age of 16. He mustered in as a private in Company G of the 1st USCT Infantry Regiment later that day. His enlistment records describe him as 5 feet, 3 1/2 inches tall, with black hair, black eyes, and a black complexion. The regiment took part in several key operations throughout Virginia and North Carolina. The men served in Norfolk, Portsmouth, and Yorktown before taking part in the Siege of Petersburg. They took part in the Battle of the Crater on July 30, 1864, and the Battle of New Market Heights and Chaffin's Farm in late September. In December 1864, they carried out an expedition to Fort Fisher, North Carolina. 

The regiment then participated in a number of actions throughout North Carolina, including the capture of Fort Fisher on January 15, 1865, and the occupations of Goldsboro and Raleigh that spring. The men were present at the surrender of General Joseph E. Johnston at Bennett Place on April 26, and the army then assigned them to duty in the Department of North Carolina until September. Walker mustered out with his regiment on September 29, 1865, at Roanoke Island, North Carolina, and then disappeared from the historical record.