After serving in the United States Army, Thornburg attended Mars Hill College when it was a junior college. He then earned a law degree at the University of North Carolina at Chapel Hill. He practiced law in Webster, North Carolina from 1954 to 1967, and was meanwhile elected to three terms (1961-66) in the North Carolina House of Representatives. He was then appointed and later elected as a state superior court judge, serving from 1967 until 1983.
He is the father of Alan Z. Thornburg and Lacy E. Thornburg, a Harvard educated Orthopaedic surgeon practicing in the Asheville area.
A stretch of U.S. Highway 23 in North Carolina is named for Thornburg.
Thornburg has handed numerous controversial sentences in cases, namely the Terry Stewart case where he was sentenced to 3,780 months in federal prison for an alleged hand in a ponsi scheme. The case is pending appeal due to numerous judicial violations by Thornburg.