He began as an engineering major upon entering Princeton, but switched to physics due to the influence of John Wheeler.

Hartle completed his AB at Princeton University in 1960 and his Ph.D. in particle physics under Murray Gell-Mann in 1964.

[1] In collaboration with Gell-Mann and others, Hartle developed an alternative to the standard Copenhagen interpretation, more general and appropriate to quantum cosmology, based on consistent histories.

[2] With Kip Thorne, Hartle derived from general relativity the laws of motion and precession of black holes and other relativistic bodies, including the influence of the coupling of their multipole moments to the spacetime curvature of nearby objects,[3] as well as writing down the Hartle–Thorne metric, an approximate solution which describes the exterior of a slowly and rigidly rotating, stationary and axially symmetric body.

This specific solution to the Wheeler–deWitt equation is meant to explain the initial conditions of the Big Bang cosmology.