I Marine Expeditionary Force is the largest of the three MEFs in the Fleet Marine Force and is often referred to as the "Warfighting MEF" for its consistent involvement and contributions in major armed conflicts.

It is presently commanded by Lieutenant General Michael Cederholm.

The deputy commander is Brigadier General Michael R. Nakonieczny.

The convention of using Roman numerals to designate a MEF, which is itself the Marine Corps equivalent organization to an Army corps, stems from the U.S. Army practice that began in the American Civil War, and continues today, of numbering corps (two or more divisions with supporting troops, and sometimes including separate brigades, regiments, groups, or battalions, all under a unified corps headquarters, usually commanded by a lieutenant general) with Roman numerals.

When directed, I MEF deploys and is employed as a Marine Air Ground Task Force (MAGTF) in support of Combatant Commander (COCOM) requirements for contingency response or Major Theater War; with appropriate augmentation, serves as the core element of a Joint Task Force (JTF); prepares and deploys combat-ready MAGTF's to support COCOM presence and crisis response; and supports service and COCOM initiatives as required.