Before King's, he was Professor of Philosophy, Dean of the School of Theology & Missions, and Vice President for Spiritual Life at Union University in Jackson, Tennessee.

[5] In 1996, Thornbury received a Master of Divinity from Southern Seminary, and subsequently enrolled in the Ph.D. program at Southern with an emphasis in philosophical theology under the supervision of R. Albert Mohler,[6] and completed additional graduate study at Green College in Oxford (now Green-Templeton College).

[10] In 1998, David Dockery of Union recruited him to teach philosophy,[4] and he received tenure in 2006 after being promoted to associate professor.

He also supervised Union's Spiritual Life division as vice president, and led the university's chapel, and campus discipleship programs.

[16] He has also been active in the arts, and has spoken at exhibitions of the work of Carl Perkins, Larry Norman, and Daniel Johnston.

The announcement was seen as a return to King's founding vision as a Christian, evangelical college and a repudiation of the political direction it had taken during Dinesh D'Souza's tenure.

[27] Thornbury has since accepted the position of Vice President at the New York Academy of Art and resigned as Chancellor of The King's College in 2018.

He has contributed to volumes such as A Theology for the Church, Faith and Learning: A Handbook for Christian Higher Education, The Lord's Supper: Remembering and Proclaiming Christ Until He Comes.

[29] His book on Larry Norman has received widespread praise including a substantially positive mention in an article in The New Yorker magazine written by Kelefa Sanneh.