The F-84B, which differed from YP-84A only in having faster-firing M3 machine guns, became operational with 14th Fighter Group at Dow Field, Bangor, Maine in December 1947. Flight restrictions followed immediately, limiting maximum speed to Mach 0.8 due to control reversal, and limiting maximum acceleration to 5.5 g (54 m/s²) due to wrinkling of the fuselage skin. To compound the problem, parts shortages and maintenance difficulties earned the aircraft the nickname, "Mechanic's Nightmare". On 24 May 1948, the entire F-84B fleet was grounded due to structural failures.
A 1948 review of the entire F-84 program discovered that none of the F-84B or F-84C aircraft could be considered operational or capable of executing any aspect of their intended mission. The program was saved from cancellation because the F-84D, whose production was well underway, had satisfactorily addressed the major faults. A fly-off against the F-80 revealed that while the Shooting Star had a shorter takeoff roll, better low altitude climb rate and superior maneuverability, the F-84 could carry a greater bomb load, was faster, had better high altitude performance and greater range. As a temporizing measure, the USAF in 1949 committed US$8 million to implement over 100 upgrades to all F-84Bs, most notably reinforcing the wings. Despite the resultant improvements, the F-84B was withdrawn from active duty by 1952.
The F-84C featured a somewhat more reliable J35-A-13 engine and had some engineering refinements. Being virtually identical to the F-84B, the C model suffered from all of the same defects and underwent a similar structural upgrade program in 1949. All F-84Cs were withdrawn from active service by 1952.
The structural improvements were factory-implemented in the F-84D, which entered service in 1949. Wings were covered with thicker aluminum skin, the fuel system was winterized and capable of using JP-4 fuel, and a more powerful J35-A-17D engine with 5,000 lbf (22.24 kN) was fitted. It was discovered that the untested wingtip fuel tanks contributed to wing structural failures by inducing excessive twisting during high-"g" maneuvers. To correct this, small triangular fins were added to the outside of the tanks. The F-84D was phased out of USAF service in 1952 and left Air National Guard (ANG) service in 1957.
The first effective and fully capable Thunderjet was the F-84E model which entered service in 1949. The aircraft featured the J35-A-17 engine, further wing reinforcement, a 12 in (305 mm) fuselage extension in front of the wings and 3 in (76 mm) extension aft of the wings to enlarge the cockpit and the avionics bay, an A-1C gunsight with APG-30 radar, and provision for an additional pair of 230 gal (870 L) fuel tanks to be carried on underwing pylons. The latter increased the combat radius from 850 to 1,000 miles (740 to 870 nmi; 1,370 to 1,610 km).
One improvement to the original F-84 design was rocket racks that folded flush with the wing after the 5-inch HVAR rockets were fired, which reduced drag over the older fixed mounting racks. This innovation was adopted by other U.S. jet fighter-bombers.
Despite the improvements, the in-service rates for the F-84E remained poor with less than half of the aircraft operational at any given time. This was primarily due to a severe shortage of spares for the Allison engines. The expectation was that F-84Es would fly 25 hours per month, accumulating 100 hours between engine overhauls. The actual flight hours for Korean War and NATO deployments rapidly outpaced the supply and Allison's ability to manufacture new engines. The F-84E was withdrawn from USAF service in 1956, lingering with ANG units until 1959.
The definitive straight-wing F-84 was the F-84G which entered service in 1951. The aircraft introduced a refueling boom receptacle in the left wing, autopilot, Instrument Landing System, J35-A-29 engine with 5,560 lbf (24.73 kN) of thrust, a distinctive framed canopy (also retrofitted to earlier types), and the ability to carry a single Mark 7 nuclear bomb. The F-84G was retired from USAF in the mid-1960s.
Starting in the early 1960s, the aircraft was deployed by the Força Aérea Portuguesa (FAP) during the Portuguese Colonial War in Africa. By 1972, all four operating F-84 aircraft were supplementing the FAP in Angola.