Horace Maynard was born in 1814 in Westboro, Massachusetts, and taught at the Millbury Academy before graduating from Amherst College in 1838. Maynard then moved to Tennessee, where he worked as a tutor at East Tennessee College. He then became a professor of mathematics and mechanical philosophy at the College in 1842.

Two years later, Maynard was admitted to the state bar and then opened a law practice in Knoxville. In 1853, he made a failed bid as a Whig for a seat in the U.S. Congress. Three years later, he had left the Whig Party to become a member of the Know-Nothing, or American, Party. It was as a member of the latter group that he was elected in 1857 to the U.S. House of Representatives, where he served until 1863. During this time, Maynard was forced to switch political allegiances when the American Party dissolved.

In 1861, Maynard switched parties yet again when he became a Unionist. During the Civil War, he worked to keep Tennessee in the Union. He also assisted the military governor of Tennessee, Andrew Johnson, as state attorney general, serving in that position from 1863 to 1865.

In 1866, Maynard returned to Congress and served in the House. He made a failed bid to become governor of Tennessee (1874), and instead became U.S. minister to Turkey at the request of President Ulysses S. Grant. In 1880, President Rutherford B. Hayes tapped Maynard to become his postmaster general following the resignation of David M. Key. After leaving the cabinet in 1881, Horace Maynard returned to Tennessee, where he died in 1882.