The circuit was a triangular layout of public roads south and west of the city of Le Mans, forming a closed course entirely on routes that remained open to everyday traffic outside of race events. In the 1920s the roads were unpaved in places and the route did not yet pass through the sections of track that would later become iconic — the Dunlop Bridge, Tertre Rouge, and the Mulsanne Straight had not yet been incorporated into the layout.
The original 1921 configuration ran from a point near the present pit facilities down into the city itself, passing through a sharp right-hand corner near the river Huisne at the Pontlieue bridge — a hairpin subsequently removed from the circuit in 1929. The course then departed the city along the section now named Avenue Georges Durand, continuing south toward Mulsanne. At 17.261 km this inaugural layout was also unpaved for significant portions.
The 1921 French Grand Prix, formally the XV Grand Prix de l'Automobile Club de France, was held on 25 July 1921 over 30 laps of the circuit for a total race distance of 517.8 km. It was the first French Grand Prix since 1914, the pre-war series having been halted by the outbreak of the First World War.
Engine regulations for the race mirrored those of the Indianapolis 500, with a maximum displacement of 3 litres. German constructors were excluded from entry. The field comprised entries from the United States (Duesenberg), France (Ballot, Mathis), and the British-French Sunbeam-Talbot-Darracq alliance, though planned Fiat entries from Italy did not materialise due to labour disputes.
Cars were not sent off in a massed start; instead they were released in pairs at one-minute intervals. American Jimmy Murphy, driving a Duesenberg fitted with hydraulic brakes — a significant technical advantage over the mechanically braked European cars — took victory after a hard-fought contest. The Duesenberg's superior braking capability helped Murphy manage tyre wear more effectively than the Ballot runners, whose cars cornered better but suffered on the long straights. Murphy's win made him the last American to win a major European Grand Prix until Dan Gurney's Eagle victory in the 1967 Belgian Grand Prix, and the last American constructor to win in Europe until Ford at the 1966 24 Hours of Le Mans.
The Le Mans circuit underwent significant revision in the years following 1921. A bypass within the city shortened the track in 1929, and the troublesome Pontlieue hairpin was eliminated at the same time. The route was bypassed around the city completely in 1932 with the addition of the section from the pit area via the Dunlop Bridge and the Esses to Tertre Rouge. This classic 13.492 km configuration, already well established as the 24 Hours of Le Mans circuit by that point, remained substantially unchanged for decades.
The 1921 French Grand Prix was the first major post-war motor race on the Sarthe circuit and marked the debut of a venue that would go on to host the world's most celebrated endurance race. Murphy's win for Duesenberg was a landmark result in transatlantic motor racing history, and the circuit's triangular public-road character typified the grand-scale road racing that defined European motorsport in the interwar period.