Fort Lewis started as Camp Lewis in 1917, when the US entered World War I. Local Tacoma citizens passed a bond for two million dollars (it would be thirty-six million in today's dollars), bought seventy thousand acres, and donated the land to the Army to be used as a permanent post. This was the first outright donation of land to the US military to build a post.

The original camp was constructed in only 90 days and training of the 91st Division, originally made up of Washington State volunteers, and other units commenced immediately. At the end of the war, activity at the camp dropped drastically, to the point where locals called for the land to be returned to Tacoma for breach of contract. Funding was found, and Camp Lewis was redesignated Fort Lewis in 1927, with new units assigned there.

In the 1930s Fort Lewis expanded, adding housing and an airfield, until the end of the '30s, when the threat of war loomed, and Fort Lewis' garrison increased from about 5,000 to 37,000 troops in three years. To accommodate these soldiers the fort expanded rapidly, adding even more housing and training facilities, creating the North Fort complex.

When World War II broke out Fort Lewis became very busy in the war. As the primary Army training center of the Pacific Northwest, Fort Lewis activated, trained, and deployed six infantry divisions and assorted other battalions and smaller units over the course of the war. Fort Lewis also added the Rainier Training Area, another eighteen thousand acres south of the Nisqually River, '43, and Madigan Army Medical Center '44. From 1942 to 1946, the Fort was also the site of a camp for German POWs, who were kept occupied with work details and recreation.

With the end of the war Fort Lewis activity dropped again, but the Cold War brought activity back up again within a few years. Fort Lewis has a vital strategic position in the Northwest, well situated for defense or deployment to east Asia, and Fort Lewis has been at the forefront of shipping troops to the Korean War, Vietnam War, Gulf War, and the Iraq and Afghanistan wars. Since 1981 Fort Lewis has been HQ for I Corps. Following this the 1st Special Forces group was activated in 1984, the 35th Air Defense Artillery Brigade in 1985, and the 66th Aviation Brigade and the 201st Military Intelligence Brigade in 1987. The 1990's saw the activation of the 1st Personnel Group at Fort Lewis, as well as the 555th Engineer Group and the 210th Field Artillery Brigade. As a result of the 2005 BRAC commission, Fort Lewis became a Joint Base with the McChord Air Force base in 2010. The base continues to serve as a training and mobilization center for all services.