The car was designed by ex-March engineer John Clarke and first appeared at the 1973 Spanish Grand Prix as the Iso–Marlboro IR. The initials IR stood for Iso Rivolta, though the Italian sports car manufacturer had no involvement in the car's actual design. The chassis was a conventional Formula One design for the era, featuring a wedge shape with a low centre of gravity in the manner pioneered by the Lotus 72 and McLaren M23. It used double wishbone front suspension with upper and lower links at the rear and outboard springs all around. Fuel tanks were positioned on either side of the cockpit. During the 1973 season, Italian engineer Giampaolo Dallara revised the rear suspension, and Ron Tauranac made further development changes. One of the original chassis was destroyed in an accident at the 1973 German Grand Prix and a replacement was built, retaining the same chassis number "02."
When sponsors Iso and Marlboro both withdrew before the 1974 season, the cars were renamed the Iso–Marlboro FW, with Frank Williams retaining the FW initials he would carry throughout his career. A third chassis, numbered "03," was completed during the 1974 campaign. For 1975, after Frank Williams fully rebranded as Williams Grand Prix Engineering, the three surviving cars were renamed FW01 (original chassis "01"), FW02 (chassis "02"), and FW03 (chassis "03"), though all three were mechanically the same design, revised for 1975 by Ray Stokoe.
The IR made its debut at the 1973 Spanish Grand Prix driven by Howden Ganley and Nanni Galli, who qualified on the last two rows of the grid. Galli finished 11th while Ganley retired from fuel starvation. The season proved difficult, but a notable result came at the Canadian Grand Prix, where Ganley led the race for eight laps after unusual circumstances involving the debut deployment of a safety car in Formula One history. Ganley eventually finished sixth, with some lap charts controversially suggesting he may have won. The car scored two World Championship points across 1973, leaving the team tenth in the Constructors' Championship. Gijs van Lennep scored the team's first ever championship point with a sixth-place finish at the Dutch Grand Prix, while Jacky Ickx finished seventh at the United States Grand Prix.
For 1974, with reduced sponsorship and financial pressures, Frank Williams ran a smaller operation. Arturo Merzario was the lead driver, joined at various races by Tom Belsø, Gijs van Lennep, Richard Robarts, and Jacques Laffite. The season's best result came at the Italian Grand Prix, where Merzario finished fourth to score three World Championship points. In South Africa, Merzario qualified an impressive third on the grid. The team accumulated four points across the season, again finishing tenth in the Constructors' Championship.
Entering 1975, the rebranded Williams team retained Merzario and Laffite as its drivers. By now the FW design was genuinely outdated, and the team was so cash-constrained it was occasionally forced to purchase used tyres from other teams. The FW01 (chassis "01") was used only as a spare throughout 1975 and never started a World Championship race under that designation. The FW02 was used for the first three races before being retired. The FW03 served as the primary race car until the replacement FW04 was ready from the Spanish Grand Prix onward. Laffite achieved a surprise second place finish in the new FW04 at the British Grand Prix, bringing a much-needed financial boost to the team. All six points Williams scored in 1975 came from the FW04, not the older chassis.
In 1976, Frank Williams sold the FW03 to Swiss privateer Loris Kessel, who heavily modified the chassis, renamed it the Apollon, and attempted to enter it at the 1976 and 1977 Italian Grands Prix. Neither attempt resulted in a race start. The FW01 and FW02 chassis did not race again in Formula One after the 1975 season. The whole FW series represented a transitional period in Williams history — a struggling backmarker operation running recycled machinery on minimal funding, before Frank Williams rebuilt around the FW04 and ultimately found the team's competitive footing in subsequent seasons.
The Iso–Marlboro/Williams FW cars hold a unique place in Williams history as the team's earliest competitive machinery. While results were modest, the car produced some memorable individual moments, including Ganley's improbable Canadian Grand Prix lead, Merzario's strong qualifying run in South Africa, and a fourth-place finish at Monza. The chassis also marked the first appearances of several prominent drivers in Williams machinery, including Jacques Laffite, who would return to the team later in his career. The gradual transition from the Iso–Marlboro branding to the Williams name across these three cars traces the earliest evolution of what would become one of Formula One's most successful constructors.