William Milmine was born around 1840 in New York. He grew up in Amsterdam, New York, with his grandparents William and Margaret Milmine. His grandfather worked as a farmer, and by 1850, he owned $1,000 of real estate. By 1860, Milmine was working as a farmer in Amsterdam. He enlisted in the Union army on April 25, 1861, and he mustered in as a private in Company b of the 32nd New York Infantry on May 31. The regiment took part in the First Battle of Bull Run, the Peninsula campaign, and the Battle of Antietam. He mustered out on June 9, 1863. He died sometime after 1863.