Thomas M. Gunn was born in Shelbyville in Shelby County, Kentucky on March 17, 1840. He was the youngest son of William and Francis Adams Gunn. William Gunn, a presiding elder of the Lexington District of the Presbyterian Church, died when his son was only thirteen years old. Thomas Gunn was still able to enter Dickinson College in Carlisle, Pennsylvania in 1858 with the class of 1860. While at the College, he was elected to the Belles Lettres Society and graduated with his class in the early summer of 1860.
Straight from Dickinson and still only twenty years old, Gunn took the post of vice-president and professor of languages at McKenzie College in Clarkesville, Texas. At the time, this Methodist institution was one of the largest in the state, although the Civil War brought its demise and it closed permanently in 1868. Gunn had already left teaching by that time, however, for he enlisted as a private in the Union Army's 21st Kentucky Infantry as soon as the war broke out. He gained a commission and served later as a unit chaplain. Following the war, Gunn embarked on a lengthy and extensive career as a Presbyterian clergyman. He was the pastor in Louisville, Kentucky in 1867. He then moved to Illinois, where he had congregations in Grand Ridge and Braidwood in the 1870s and served at Joliet from 1877 to 1885. In 1885, Gunn moved west to Walla Walla, Washington, where, in 1887, he became superintendent of missions responsible for certifying new congregations in Washington, Idaho, Oregon, and Alaska. He held this post until 1899 and then served again as a pastor in Cashmere, Washington from 1901 until his retirement.
In February 1864, Gunn married M. Catherine Waggener of Greensburg, Kentucky, and the couple had four children. On June 1, 1917, Thomas Morris Gunn died at his home in Seattle, Washington. He was seventy-seven years old.