Thornburgh was born in Pittsburgh, Pennsylvania, on July 16, 1932, the son of Alice (Sanborn) and Charles Garland Thornburgh, an engineer. Thornburgh attended Mercersburg Academy then Yale University from which he obtained an engineering degree in 1954. Subsequently, he received a law degree from the University of Pittsburgh School of Law in 1957, where he served as an editor of the "Law Review". Thornburgh was inducted into Omicron Delta Kappa at the University of Pittsburgh in 1973, and was later awarded the society's highest honor, the Laurel Crowned Circle Award, in 1996. He subsequently was awarded honorary degrees from 32 other colleges and universities. He joined the Pittsburgh-based law firm Kirkpatrick & Lockhart in 1959.