His first tour of duty was with the 1st Marine Brigade in Hawaii as a Rifle Platoon Commander, Company Executive Officer and Assistant Regimental S-3, respectively.

[3] Between his Vietnam tours, Johnston served as Commanding Officer, Guard Company, \ Headquarters Marine Corps, Washington, D.C., from July 1965 to October 1966.

Transferring to Marine Corps Base Quantico, he completed the Amphibious Warfare School in July 1968, and was subsequently assigned to Marine Corps Recruit Depot, San Diego, California, in August, where he served as Assistant G-3, Staff Secretary and Executive Officer of 2nd Recruit Training Battalion until August 1972.

[3] Following an assignment to the Joint U.S. Military Advisory Group in Korea from 1972 to 1973, Johnston completed graduate school in San Diego in March 1975.

On May 4, 1990, he was promoted to major general and on June 15, 1990 he was assigned to United States Central Command as the Chief of Staff.

In December 1992, as Commanding General of I MEF, Johnston lead U.S. and allied troops in a United Nations mandated mission, Operation Restore Hope, to deliver relief supplies to Somalia.

Lieutenant General Johnston retired on August 1, 1995 after 34 years of service in the U.S. Marine Corps.

He reportedly ran five miles every day and in his mid-fifties he achieved a perfect score on the Marine Physical Fitness Test.