The federal government closed the fort in 1868, and sold its buildings to Lucien Maxwell, a prominent New Mexico landowner, in 1870. Maxwell's son Pete befriended legendary outlaw Billy the Kid, and it was in his house that Billy was killed by Pat Garrett. Billy the Kid is buried in the old military cemetery in Fort Sumner as is Lucien Maxwell.
In the 1920s the Transcontinental Air Transport airline built an airfield in Fort Sumner as part of its coast-to-coast air passenger network, but the site was abandoned when the airline's ambitious plans collapsed in the Great Depression. The airfield was reopened by the Army Air Corps as a training base during World War II. After the war, the base became the Fort Sumner Municipal Airport, and was chosen as a launch site for NASA's high altitude balloon program.