Significant changes to the technical regulations were introduced for 2026, affecting both the chassis and power unit. These changes generally simplified the power unit, increased the share of energy generated by the electric motor compared to the combustion engine, and reduced the overall size and weight of the car, as well as the amount of downforce aerodynamic surfaces can generate. Mercedes AMG HPP, McLaren's engine supplier since 2021, faced scrutiny over the winter break when it emerged their engine exploited a measurement loophole in the power unit regulations. The 2026 regulations restricted the compression ratio of the combustion engine, which would reduce its power output. However, the compression ratio was measured by the Fédération Internationale de l'Automobile (FIA) at ambient temperatures. Mercedes developed a manufacturing technique that allowed the compression ratio to expand when the engine was hot, enabling it to produce more power while still complying with the FIA test.
Unusually, McLaren auctioned the rights to an MCL40 with RM Sotheby's before the season began. The car sold for US$11,480,000. Its owner will be able to select which driver's chassis to receive, and it will be delivered in a running condition in 2028.
The MCL40 used a one-off black and silver geometric livery for the private pre-season test at Circuit de Barcelona-Catalunya. The MCL40 livery is similar to the MCL39, featuring more orange on the engine cover and front wing, and introduces a dark green airbox and mirrors for sponsor Allwyn Entertainment. This maintained McLaren's practice of retaining the livery of a successful car for the following season. As with all McLaren cars since the MCL35, the car's livery is applied with vinyl wrap rather than paint.
Initial work on the car began in 2024. Chief designer Rob Marshall stated that McLaren would not deliver any significant changes to the car between the private test and the opening round of the season. Marshall stated McLaren's priority would be understanding how the 2026 regulation cars worked before pursuing performance improvements. Marshall identified packaging for radiators and electrical boxes as a particular challenge given the reduced size of the car.
At the Australian Grand Prix, Piastri qualified fifth and Norris sixth. Piastri did not start the Grand Prix after crashing on a reconnaissance lap, which he said was caused by "a large element of [him]", but also the energy deployment rules of the electric motor unexpectedly delivering 100 kW (130 hp) more power than expected. Norris finished fifth after battling with Max Verstappen.
The Chinese Grand Prix was McLaren's 1,000th Grand Prix entry. After finishing the sprint fourth and sixth, neither driver started the race due to power unit issues. This was the team's first double non-start since the 2005 United States Grand Prix.
At the Japanese Grand Prix, Piastri qualified third and Norris fifth. Piastri took the race lead at the start; he opted to change tyres on lap 18 to avoid the risk of Charles Leclerc undercutting him, though lost his advantage during a safety car period. Piastri finished second, and Norris fifth.
The Bahrain and Saudi Arabian Grands Prix were cancelled due to the impact of the Iran war. Stella announced that “an entirely new MCL40” would be introduced over the two North American races following the break.
Season still in progress.