In civil use, the aircraft was a quick sales success, with prominent businessmen and Hollywood actors purchasing the aircraft. In 1936, the US Navy ordered Model 24s designated as GK-1 research and instrument trainers. The type was also used by the US Army as a light transport and by the Coast Guard, with the designation J2K-1. The Civil Air Patrol operated many Fairchild UC-61/24s, and some aircraft were fitted with two 100-pound bombs for what became successful missions against German U-boats off the east coast of the United States in the early stages of the Second World War. The UC-61 was also procured by the US Navy as the GK-1 and by the British Royal Air Force as the Fairchild Argus.
Fairchild UC-61K supplied as an Argus III to the RAF in 1944 and sold to a civil owner in Belgium postwar. In 1941, the United States Army Air Forces (USAAF) placed an initial order for 163 Fairchild C-61s; however, via Lend-Lease, 161 of these were shipped abroad. Under the auspices of this program, the majority of the 525 Warner Scarab Fairchild 24s/C-61s went to Great Britain. Most of these aircraft saw service as Argus Is and improved Argus IIs and were allocated to a newly formed adjunct of the Royal Air Force (RAF), the Air Transport Auxiliary (ATA). An additional 306 Ranger-powered Argus IIIs were also used by the ATA. In British service, the majority of the Argus type operated with the ATA ferrying their aircrew to collect or deliver aircraft to and from manufacturers, Maintenance Units (MU)s and operational bases.
The Argus I was a Warner Scarab-equipped aircraft identified by its wind-driven generator located on the starboard struts, and was equipped with a black-painted propeller. The Argus II was also a Scarab-powered aircraft, usually with a transparent cabin roof. This mark was certified for heavier operational weight than the Mark I and was identified by its yellow propeller. The Argus III was equipped with the six-cylinder inverted inline Ranger engine.