William T. Leonard was born on May 27, 1837, in Massachusetts. By the early 1860s, he was living in Menasha, Wisconsin.
He enlisted in the Union army on April 20, 1861, and he mustered in as a private in Company G of the 3rd Wisconsin Infantry. He was wounded at Antietam on September 17, 1862, and again in the thigh at Chancellorsville on May 3, 1863. He mustered out on June 29, 1864.
He returned to Wisconsin after leaving the army. He applied for a federal pension in July 1865 and eventually secured one. He worked as a physician in Wonewoc, Wisconsin, and by 1870, he owned $250 of real estate and $600 of personal property. He married Martha Fisk around 1878, and she worked as a milliner. They moved to Washburn, Wisconsin, in the 1880s and then to Campbell, Minnesota, in the 1890s. By 1910, he was living in Farmington, Wisconsin. He died on March 3, 1918.