About 100 Marines and sailors began working on the island this month under an agreement with the I Marine Expeditionary Force at California’s Camp Pendleton and the Catalina Island Conservancy.
The work on Catalina’s Airport in the Sky is paid for by $5 million donated to the nonprofit land trust.
However, the group isn't paying the Marines and sailors for the work, which the military considers valuable training.
The conservancy had been patching the runway at a cost of about $250,000 a year until the state Department of Transportation's aeronautics division said it needed a long-term repair plan.
The trust then partnered with the Defense Department’s Innovative Readiness Training Program, which looks to match up the needs of communities with military training opportunities.
The airport closed in December, when tons of construction supplies were sent over from the mainland.
The military set up an encampment at the airport and began work to replace the existing asphalt with concrete.
The airport dates to 1941, when it was built by leveling two mountaintops and filling in the remaining canyon to create the 3,000-foot (914-meter) main runway, which sits at an elevation of about 1,600 feet (488 meters) about 10 miles from the city of Avalon.
One end of the runway has a huge drop-off and using it has been likened to landing on an aircraft carrier.
The runway is expected to reopen in April.
The renovation is expected to give the runaway a 75-year life.
Catalina lies south of Los Angeles, within sight of the mainland, and ferries bring most of the visitors.
Efforts at making the island a resort date to the 1800s but took off the 1920s when chewing-gum magnate William Wrigley Jr. began extensive development of facilities and attractions, including making it the spring training home of his Chicago Cubs.