Leslie Talbot Pennington was born in spiceland, Indiana, to Quaker parents. It is likely that his time at Harvard Divinity School led him to Unitarianism. Recommending him to the AUA, Willard Learoyd Sperry, the Dean at HDS wrote: "There are few men whom I have ever watched entering the ministry with a confidence and consent such as I am giving Mr. Pennington. He is a man of transparent sincerity and integrity, of very great personal charm, and of real distinction of mind."

Pennington served congregations in Lincoln and Braintree, Massachusetts, Ithaca, New York, Chicago, Illinois, and his last call was at WestNewton, Massachusetts.

He served on the AUA Board (Chairing the Ministry and Fellowship committees), USC Board, and chaired the 1947 COmmission on Church Union - amongst many other groups.

He was a noted peace activist throughout his career and was a founder of the Greenfield Group.