William Thomas was born in Plymouth, Massachusetts, on March 15, 1789, the son of Joshua Thomas and Isabella Stevenson. After studying privately with a local minister, he entered Harvard University in 1803, at the age of 14. Following his graduation in 1807, he studied law with his father, and then worked as a lawyer in Plymouth for the rest of his life. During the War of 1812, he served as quartermaster of the Massachusetts Militia's 1st Regiment, 5th division. He held many political positions throughout his life, including justice of the peace, master of chancery, county sheriff, and member of the Massachusetts legislature. He and his wife, Sarah Warren Sever (d. 1871) had one daughter, Ann, and cared for their grandchildren after Ann's death in 1855. At the time of his death on September 20, 1882, William Thomas was one of the oldest living graduates of Harvard University and the oldest living resident of Plymouth, Massachusetts.