The mission of the MEU is to provide geographic combatant commanders with a forward-deployed, rapid-response force capable of conducting conventional amphibious and selected maritime operations at night or under adverse weather conditions from the sea, by surface and/or by air while under communications and electronics restrictions.

Since its activation in July 1987, the 15th MEU has trained to meet its mission in the rotation with the 11th and 13th MEUs to provide a continuous presence in the Western Pacific, Indian Ocean, and Persian Gulf and as a ready MAGTF in the continental United States.

In October 1989, the MEU assisted in relief efforts following the 1989 Loma Prieta earthquake, performing their assigned missions and also taking on several volunteer projects to help the victims of the disaster.

Assistance lasted over a month as the Marines distributed food and medical supplies, evacuated stranded villagers and provided security to other rescue personnel.

[3] After a predawn landing, the Marines secured the capital city of Mogadishu, the international airport and maritime shipping port facilities, as well as the American Embassy, and quickly moved into other inland areas to protect food distribution convoys and patrol the streets to restore order.

On 7 October 2001, the 15th MEU participated in the United States' new "War on Terrorism", sending Marines and Sailors into Northern Pakistan to establish a forward operating air base and logistical hub.

Landing at a remote airbase, 90 miles (140 km) southwest of Kandahar, the Marines occupied Camp Rhino that had been secured by the U.S. Army special forces, America's first Forward Operating Base while maintaining the first significant conventional ground presence in Afghanistan.

In mid-February, elements of the MEU off-loaded and established a training camp in Northern Kuwait while other members of the MAGTF – primarily the helicopter squadron – remained on board the Tarawa ARG in the Persian Gulf.

During February and March 2003, tactical control (TACON) of the MEU was assigned to the United Kingdom's 3 Commando Brigade Royal Marines for Operation Iraqi Freedom.

On 21 March 2003, Marines from the 15th MEU crossed the border into Southern Iraq and secured the ports of Umm Qasr and Az Zubayr in order to destroy Iraqi resistance and enable follow-on humanitarian assistance to begin.

In January 2005, the 15th MEU participated in Operation Unified Assistance by providing disaster relief to survivors of the destructive tsunami in Sumatra, Indonesia and southern Sri Lanka.