He was a Special judge of the Superior Court of the 30th Judicial District of North Carolina from 1967 to 1971.

He was a Resident judge of the Superior Court of the 30th Judicial District of North Carolina from 1971 to 1983.

He was defeated in the primary by former governor Jim Hunt, who went on to win the general election.

Thornburg was nominated by President Bill Clinton on January 11, 1995, to a seat vacated by Robert D. Potter.

He was confirmed by the United States Senate on March 17, 1995, and received commission the same day.