She was sponsored by Leonor K. Sullivan, Representative from the 3d District of Missouri and commissioned on 22 November 1969 at the Norfolk Naval Shipyard, Captain John W. Klinefelter in command.

St. Louis spent May and June in post-shakedown availability and the greater part of July in provisioning preparatory to her first deployment with the fleet.

St. Louis completed the amphibious exercise in early January, spent 15 days in upkeep in Subic Bay, then headed north again for two months of shuttling men and cargo between Vietnam, Okinawa, and Japan.

After a month and a half stand down period in Long Beach and three more weeks of local operations and upkeep there, she returned to Vietnam, arriving in Da Nang on 24 June.

On 31 March 1972 St. Louis headed out of Long Beach Naval Shipyard back to the picket line off the coast of South Vietnam, participating in the defense of Quảng Trị Province during the Easter Offensive on 24 May 1972.

The St. Louis offloaded South Vietnamese Marines and US Navy SEAL squads during this assault, earning a campaign star, and later, in the 1990s, the Combat Action Ribbon.

After seven months of transporting men and cargo between various bases in the western Pacific, she returned to Long Beach on Veterans Day 1972.

She sailed to several countries Philippines, Thailand, Singapore, Africa, Freemantle, Australia, Okinawa, South Korea.

In 1983 she changed home port to Sasebo, Japan where she performed troop transfers between Okinawa and Korea and amphibious ready group deployments until she was decommissioned in 1992.

On board were the personnel and equipment of Charlie Company, 9th Engineer Support Battalion, and Marines from ELMACO 3rd FSSG whose 4-month deployment was ended earlier than scheduled due to the ship's new orders.

At Naha, the St. Louis would load 28 reverse osmosis water purification units ( ROWPUs) each weighing more than 5 tons for use in the relief effort.