Kernan's initial assignment was as liaison officer with the 1st Brigade, 82nd Airborne Division, Fort Bragg, North Carolina.

Following this assignment he returned to Fort Bragg, North Carolina, and served as company commander, 2d Battalion (Airborne), 325th Infantry.

In August 1981, he was selected to be the U.S. Exchange Officer representing the 82d Airborne Division to the British Parachute Regiment where he commanded Company C, 3 Para, for two years.

On December 20, 1989, he led the 75th Ranger Regiment in its combat parachute assault into Panama during Operation Just Cause.

In July 1993, he was assigned as the director of plans, policy, and strategic assessments, J5, United States Special Operations Command, MacDill Air Force Base, Florida.