Clark earned his Bachelor of Arts in Theology from Maryknoll College in 1966, his Doctor of Philosophy in philosophy from Columbia University in 1971, and his Juris Doctor from Harvard Law School in 1972. From 1972 to 1974, Clark was an associate with the Boston law firm of Ropes and Gray, where he practiced commercial and corporate law. After this, Clark spent four years on the faculty of Yale Law School, where he became a tenured professor. In 1979, he returned to Harvard Law School as a professor of law.