The MCL35 had been designed for Renault power under lead designer James Key, who had been recruited from Scuderia Toro Rosso in 2019. The 2020 car enabled McLaren to finish third in the Constructors' Championship for the first time since 2012, using a design philosophy emphasising the car as a whole rather than the performance of individual components.
For 2021, the FIA's token-based development restriction system limited how much teams could change from their 2020 cars. However, because McLaren had already contracted Mercedes engines before the freeze was announced, they received special dispensation to modify the chassis to accept the new power unit. The change of engine supplier required redesigning the hydraulics, cooling, electronics, pipework, and control boxes, since the Mercedes M12 engine placed the compressor at the front and the turbine at the rear — opposite to the Renault E-Tech 20's layout which positioned both at the rear. The gearbox was also new, and its different dimensions lengthened the car's wheelbase. The MCL35M was consequently the only car in 2021 required to re-homologate its chassis with the FIA.
McLaren's technical team described the parts count as equivalent to building a new car entirely. James Key identified two primary development targets: low-speed cornering performance and wind sensitivity, both areas where the MCL35 had been weakest. The MCL35M passed FIA crash tests in December 2020 and had its first shakedown at Silverstone on 16 February 2021.
Pre-season testing at Bahrain produced encouraging lap times without major reliability problems. In the opening race, Norris finished fourth and Ricciardo seventh, giving McLaren third place in the early Constructors' standings behind Red Bull and ahead of Ferrari.
Ricciardo's adaptation to the MCL35M proved difficult across the season. The car demanded an unusual overlap between the braking and cornering phases, particularly at low-speed corners — a characteristic that did not suit the Australian's natural driving style. Norris, by contrast, generally extracted strong results and challenged for podiums throughout the year.
The season's highlight came at the Italian Grand Prix at Monza, where McLaren produced their best result in years. Ricciardo led Norris to the finish line after Max Verstappen and Lewis Hamilton collided during pit stop sequences, handing the team their first race win since the 2012 Brazilian Grand Prix and their first one-two since the 2010 Canadian Grand Prix. Ricciardo also set the fastest lap on the final tour.
Norris delivered several other strong performances, including a front-row start in Austria and pole position at the Russian Grand Prix — the first pole for a McLaren since 2012. In Russia, Norris led the race comfortably until light rain began with a handful of laps remaining. Declining McLaren's recommendation to switch to intermediate tyres, Norris slid off track and dropped from first to seventh before finally pitting. The incident became one of the defining moments of his 2021 season.
McLaren ran one-off liveries for two races: the Gulf Oil colours — light blue with orange stripe — for the Monaco Grand Prix, and a Vuse-partnership design at the Abu Dhabi finale. The Gulf livery proved particularly popular and directly influenced the MCL36's permanent colour scheme for 2022.
McLaren finished the season fourth in the Constructors' Championship, losing third place to Ferrari, while Norris placed sixth and Ricciardo eighth in the Drivers' standings.
The MCL35M was characterised by its strong straight-line performance and effective power deployment, making it difficult to overtake according to rival drivers. Its Achilles heel was mid-corner performance, particularly at low-speed sections, where Ferrari's SF21 had an advantage. The diffuser was a noted design innovation: by attaching inboard strakes to the floor rather than the diffuser body itself, McLaren effectively ran a larger diffuser than the regulations would otherwise permit, a concept that technical journalists described as the most original aerodynamic innovation of the 2021 season.
The MCL35M continued as a test vehicle well into 2022 and 2023 as part of McLaren's Testing of Previous Cars programme. Among those who drove it were Colton Herta, Alex Palou, Pato O'Ward, and eventually Oscar Piastri in preparation for his 2023 race debut. The Italian Grand Prix-winning chassis is part of McLaren Racing CEO Zak Brown's personal collection.