Thomas D. Willis was born in Mill Creek Township, Pennsylvania, on April 18, 1843, the son of Thomas Willis (d. 1875) and his second wife, Marrillia M. Osborn (d. 1902). He had one full brother, Seth H. Willis (b. 1847), and four surviving half siblings: Frank, Matilda, Martha, and Edwin. Willis served in the 15th Pennsylvania Cavalry Regiment, Companies C and F, from August 22, 1862-June 21, 1865. In early 1863, he was imprisoned in Nashville in connection with the regiment's attempted mutiny, and in late 1864, he was hospitalized because of a large open sore on his hip. After the war, Willis returned to Mill Creek, where he ran the family farm until becoming president of the Erie County Milk Association in 1901. He and his wife, Anna J. McKee, married on September 26, 1876, and had four children: Gussie B., Thomas C., Harvey M., and Frederick. Thomas D. Willis died on May 30, 1920.