In 1926, the U.S. Congress authorized the Secretary of War to sell surplus military reservations, including Forts Dade and De Soto. The state, county, and city governments chose not to purchase the property for the appraised price of $192,000. Therefore, the land was offered for public sale by sealed bid to be opened April 16, 1928. There were two bids and both were rejected.
Many storms had hit Fort De Soto during its short life including hurricanes in October 1921, September 18, 1926, and September 4, 1935. In October of 1932, there were still twenty-six of the original twenty-nine post buildings standing even after the storms. However, the average estimated value of each had fallen to $250, or a total of $6,000 compared to the original construction cost of over $120,000. Also that same month, Battery Bigelow collapsed and was swallowed by the Gulf of Mexico.
The United States Army was not the only governmental agency to occupy Mullet Key. The Hillsborough County Board of Health maintained a quarantine station from December 16, 1889, until May 1899. The Secretary of the Treasury transferred 271 acres of the eastern end of Mullet Key to his department for quarantine purposes in May 1899. There were four buildings removed from Egmont Key and rebuilt on an area of Mullet Key adding to the existing cottage and sanitary facilities. In 1901, the Marine Hospital Service took over jurisdiction of the station from the Florida State Board of Health. In 1902, the agency's name was changed to Public Health and Marine Hospital Service. The duty of the station was to inspect aliens aboard ships from foreign ports. The station had fifteen buildings in 1925. The mosquitoes were a problem for this organization as well. In 1933, the Public Health Service was given permission to begin mosquito eradication on the portion of Mullet Key that the Army had occupied. Their program was a success.
The Mullet Key Quarantine Station was no longer needed when the Public Health Service moved their headquarters to Gadsden Point, outside of Tampa, in 1937. On September 29, 1938, the Pinellas County Board of County Commissioners bought the 271-acre tract on Mullet Key for $12,500.
The War Department made a decision in 1940 to turn Mullet Key into a bombing range. The Army negotiated with the Department of the Interior and Pinellas County, and the tract was returned to military status and became a subpost of MacDill Field in June 1941.