The F9F-2 and -3 initial production versions of the Panther were declared service-ready in the spring of 1949. However, because of initially-slow deliveries of the Pratt & Whitney J42, the Allison J33-powered F9F-3 was actually the first to enter service. F9F-3s were first delivered to VF-51 at NAS San Diego on May 8, 1949. The F9F-2s first went to the Blue Angels flight demonstration team based at NAS Pensacola on August 20, 1949, and a few days later F9F-2s went to VMF-115 at MCAS Cherry Point in North Carolina. The first Navy squadron to get the F9F-2 was VF-11 at NAS San Diego, which first received the machines in the early autumn of 1949.
When war came to Korea on June 25, 1950, VF-51 and VF-52 loaded their F9F-2 Panthers aboard the USS Valley Forge (CV-45) and put to sea. Panthers from VF-51 were first in action on July 3, 1950, providing escort for a strike against an airfield at Pyongyang. Ens E. W. Brown and Lt(jg) L. H. Plog shared credit for downing a Yak-9, scoring the first kill credited to a Navy jet fighter.
In November of 1950, the swept-wing MiG-15 began to appear over northern Korea. Although powered by derivatives of the same Rolls-Royce Nene as was the F9F-2, the MiG-15 had a much better performance because of its swept wings and lighter weight. Nevertheless, the Panther was able to defeat the MiGs in the few air-to-air encounters that did take place. The first MiG-kill by a F9F Panther was by Cdr W. T. Amen of VF-111 on November 9, 1950. Four more MiG-15s were downed by Panthers before the Korean War ended, with no Panthers being lost in air-to-air combat.
The first Marine Corps Panthers appeared in combat in Korea in December of 1950. These were F9F-2Bs serving with VMF-311, helping to support the withdrawal of troops from the Chosin Reservoir.
Most Panthers serving in Korea flew air-to-ground close-support missions, air-to-air action being quite rare. The first Navy F9F-2Bs appeared in combat on April 2, 1951, when VF-191 flew an attack on railway bridges near Songjin.
The more capable F9F-5 first appeared in Korea in October of 1952, flown initially by VF-781 and VF-783 operating off the USS Oriskany (CVA-34). F9F-5s also served with VF-51, VF-52, VF-53, VF-111, VF-153, and VF-154.
The Marine Corps flew F9F-2Bs, F9F-4s and F9F-5s in Korea, exclusively in the ground support role. Active and reserve Marine Corps squadrons that flew the Panther included VMF-115, VMF-122, VMF-211, VMF-213, VMF-214, VMF-223, VMF-224, VMF-232, VMF-234, VMF-235, VMF-311, VMF-312, VMF-314, VMF-324, VMF-334, and VMF-451, plus the training squadrons VMFT-10 and VMFT-20.
F9F-2P unarmed reconnaissance aircraft were deployed by VC-62 aboard the USS Princeton in December of 1950. Two years later, they were replaced by F9F-5Ps. Two Marine Corps reconnaissance (VMJ-1 and VMJ-3) squadrons flew F9F-5Ps. VMJ-3 was still flying F9F-5Ps when the squadron was redesignated VMCJ-3.
Throughout the early 1950s, Panthers served extensively with active and reserve units of both the Atlantic and Pacific Fleets. They provided the mainstay of the Navy's jet-powered carrier-based air-to-ground capability. The Panthers were phased out of active service with the Navy in 1956, but they remained with training units until 1958. The last Marine Panther units were the two reserve squadrons VMF-213 and VMF-234, which were based at Minneapolis. These Panthers were retired in 1958.
Many of the surplus Panthers were used as drones or as drone directors under the designation F9F-5K or F9F-5KD. In 1962, the Defense Department eliminated separate designations for Navy aircraft, and ordered that all Navy planes be redesignated under the new Tri-Service unified designation scheme. The Panther/Cougar was assigned the designation F-9 under the new system. By this time, the only Panthers left in service were the F9F-5KD drone directors. These were redesignated DF-9E. The last of these DF-9Es was stuck off charge in the mid-1960s.