Constructed almost entirely of aluminum and fiberglass, the ship's light weight and gas turbine engines made the patrol gunboat well suited for counterinsurgency and coastal blockade operations.

On 28 August, Rear Admiral Elmo R. Zumwalt, (prospective Commander, Naval Forces, Vietnam), rode the ship during a patrol problem to gain insight into the gunboat's capabilities.

In November, the gunboat entered Long Beach Naval Shipyard for a short post-shakedown availability, a period lengthened by labor and material problems to 5 March 1969.

Less than a month later on 15 April, Marathon suffered an engine casualty and more delays as yard workers had trouble welding the gunboats' aluminum deck plates.

Following a tactical exercise in January 1970, and coastal interdiction training in March, Marathon departed San Diego on 2 June for her new home port at Guam in the Marianas Islands.

One of Marathon's first tasks was to escort battle damaged USS Canon from the mouth of the Bo De River back to Cam Ranh Bay on 15 July.

In January 1973, Marathon and Gallup conducted a joint week-long patrol in the Caroline Islands before resuming type training out of Guam.

Underway 13 February 1974, Marathon and other gunboats sailed to the Philippines and conducted a simulated missile boat attack on the USS Midway battle group off Subic Bay on the 19th.

Following post-repair sea trials, Marathon and the other gunboats at Guam returned to the United States, arriving in Chicago, Illinois, on 12 November after a long, circuitous transit via Pearl Harbor, San Diego, the Panama Canal, Norfolk, Virginia, the St. Lawrence Seaway and the Great Lakes.

The gunboats sailed out through the Great Lakes and the St. Lawrence River, visiting Quebec en route, before arriving in Little Creek via Newport, Rhode Island, on 25 June.