While at Rutgers, he participated in an aviation officer cadet program and was commissioned an ensign in the United States Navy.

After his release from Vietnam, he returned to college and graduated from the University of San Diego in 1976 with a BS in political science.

On June 30, his first deployment, Coker took part in a 19-aircraft attack on a radar control facility in Kép, 38 miles (61 km) northeast of Hanoi.

In the face of flak, automatic weapons fire and extremely difficult weather conditions, Coker directed the pilot, navigated his aircraft, managed the ordnance system and used his electronic countermeasures equipment to provide protection for the strike group.

His jet aircraft was diverted to attack three North Vietnamese torpedo boats in the Gulf of Tonkin that were heading towards U.S. Navy destroyers in the area.

The citation notes that "Lieutenant Coker's resourcefulness, superb airmanship, and courage in the face of great danger contributed materially to the success of the mission."

[12][13] Coker was awarded the Navy Commendation Medal for his actions as part of a coordinated 23-aircraft attack on petroleum storage tanks in Hai Phong on August 2, 1966, consisting of four waves of A-6 Intruders and A-4 Skyhawks, releasing a combination of 750 and 1,000 pound bombs and firing rockets.

Despite heavy anti-aircraft fire, Coker provided accurate guidance to the pilot on airspeed, altitude and dive angle, while operating both the attack navigation radar system and electronic countermeasures.

Despite adverse weather conditions, Coker's actions contributed to an attack that destroyed all but one of the facility's storage tanks.

[14][15] On Coker's 55th bombing mission, he and his pilot, John H. "Jack" Fellowes, were shot down over North Vietnam on August 27, 1966, while flying near Vinh in Nghệ An Province.

For three hours after they ejected, American aircraft made a thorough search, despite moderate to heavy flak, but were unable to spot the parachutes or get a signal from their emergency beepers.

[23] Coker's Silver Star citation cited his fierce resistance as one of the significant reasons the North Vietnamese eventually abandoned harsh treatment of the prisoners.

[24] Coker served as chaplain to his fellow prisoners while he was in captivity, providing them with spiritual guidance and constructive counseling.

[25] In July 1967, Coker devised "unusual and ingenious methods" to allow his fellow prisoners to communicate with each other within their camp, assisting their collective efforts to resist their captor's demands and improving the morale of American and Allied POWs.

[26] That year, Coker and Air Force Captain George McKnight (another of the "baddest") conceived an escape plan.

There they would commandeer a boat and make it out to sea where they hoped to flag down a ship of the United States Seventh Fleet, estimating that it would take eight days from their escape until their rescue.

[30] His citation for the Navy Cross reads in part: ... for extraordinary heroism during an extremely daring escape from a solitary confinement cell while a Prisoner of War in Hanoi, North Vietnam on October 12, 1967.

During a period of particularly harsh treatment, Lieutenant Commander Coker and another prisoner executed an escape as a two-man team despite the high risk of brutal reprisal or possible loss of life.

Walking under the bridge to the Red River's edge, he swam downstream all night and at sunrise buried himself in a mudbank in an effort to remain concealed.

[31][32]On October 25, 1967, Coker was one of 11 of the most effective resisters, among the 267 POWs then held, who were placed in a special facility in a courtyard behind the North Vietnamese Ministry of National Defense, soon nicknamed "Alcatraz", located about one mile away from Hỏa Lò Prison.

[37] Coker was also awarded the Meritorious Service Medal for his "aggressive leadership" as Director of the Atlantic Fleet Command Center upon retiring from the navy in September 1986.

[39] He also told the National League of Families that the circumstances of flyers lost over remote, rugged, jungle terrain meant “‘You’re going to say suddenly the likelihood of a POW in Laos is near zero.’”[40] In addition to continued Scouting activities, Coker has been involved in supporting ex–POWs for many years.

Reflecting on his wartime experiences in a Veterans Day tribute to POWs, he said "If you're never tested, you don't know ... You're capable of a great deal more than you realize, if you really set your mind to it.