[2][3] He decided early on a military career, and enlisted in the Massachusetts National Guard's 110th Cavalry Regiment in 1922, rising to the rank of sergeant and learning skills including horseback riding and polo.

During the period immediately prior to U.S. entry into World War II, Harkins participated in large-scale exercises, including the Louisiana and Pine Camp maneuvers.

In August 1942, Harkins became deputy chief of staff of Patton's Western Task Force, which was preparing for the invasion of North Africa.

As deputy chief of staff, Harkins played a major role in planning the Allied invasion of Sicily and in July 1943 he took part in the initial landings and combat at Gela.

While in that capacity, Harkins earned the nickname "Ramrod" for his determination to fulfill Patton's desire to always keep Third Army moving during combat in France.

[7] Harkins was present with Patton at the famous command and staff meeting called by General Dwight D. Eisenhower to discuss the Allied response to the German attack in the Ardennes which became known as the Battle of the Bulge, in which Patton promised that Third Army could be ready to disengage his troops from their current eastward attack and move north approximately 100 miles (160 km) to counter-attack in three days.

[8] Harkins remained in Germany after the war and took part in the occupation of Bavaria, transferring to Fifteenth Army when Patton was assigned as commander of that unit.

The head coach for Army at that time, Earl "Red" Blaik, felt that Harkins was "a black and white man with no shades of gray" and accused him of bias.

In April 1953, he was assigned as chief of staff for Eighth Army in South Korea, serving under commander Maxwell D. Taylor and receiving a promotion to major general.

Harkins was promoted to lieutenant general in 1957 and assigned as commander of NATO's Allied Land Forces, Southeastern Europe, with headquarters in İzmir, Turkey.

When details of the battle emerged that differed from the Army's official version, it became a very serious matter, and press reports of it embarrassed the Kennedy administration.

Harkins was described by Neil Sheehan as an "American General with a swagger stick and cigarette holder...who would not deign to soil his suntans and street shoes in a rice paddy to find out what was going on was prattling about having trapped the Viet Cong".

New York Times Vietnam correspondent David Halberstam became so angry with Harkins he refused to shake his hand at a Fourth of July celebration, hosted at the US Embassy, Saigon.

Mark Moyar, an associate professor at the U.S. Marine Corps University feels that Halberstam and Sheehan, along with other reporters, "horribly tarnished the reputations of some very fine Americans, including General Harkins".

[12] Moyar writes that others, such as John Mecklin (then on leave from Time as Public Affairs officer for the US embassy) observed Harkins living a "Spartan" life in Saigon and traveling "daily" by small plane around the country to gather and evaluate information from South Vietnamese and American troops.

The reason given by MacLear being that neither Kipling nor even MacArthur – no one in the history of war – had ever known the mobility and firepower that Westmoreland had been promised by Secretary of Defense McNamara, and was shortly to command.

[17] He is buried at the United States Military Academy Post Cemetery in West Point, Orange County, New York, in Section IX, Row A, Grave 053.