Due to the COVID-19 pandemic, the race was postponed from April until October to maximise the chances of a mass participation event.

The 41st running of the London Marathon was originally scheduled for April, but due to the continued effects of the COVID-19 pandemic, the race was postponed until October to increase the chances that such a large event could be held safely.

[5] The winner of each elite event received a prize of $55,000, with a total amount of $313,000 awarded to competitors based on finishing positions.

[6] Unlike the 2020 event, which was run behind closed doors on a special course around St James's Park, the 2021 race returned to the traditional London Marathon course.

[9] After winding through Canary Wharf, the route returns through Shadwell on the other side of the road to which it entered before passing through Tower Hill.

[12] The runners enter the underpass in Blackfriars before running along the Thames Embankment, past Westminster and onto Birdcage Walk.

[12][9] The course then runs parallel to St James's Park before turning onto The Mall and finishing in front of Buckingham Palace.

[12][9] The top three finishers in the 2020 elite men's race – winner Shura Kitata and runners-up Vincent Kipchumba and Sisay Lemma – all returned to compete in 2021.

Course record holder and twice former winner Manuela Schär competed, as did Tatyana McFadden, who won four times between 2013 and 2016, 2018 champion Madison de Rozario,[18] and Susannah Scaroni, who was third in the 2017 and 2018 London Marathons.

Lemma was also unable to give post-race interviews, as he was not allowed to mix with new people due to Atanaw's positive COVID test.

2020 winner Brigid Kosgei finished fourth overall;[20] she had fallen away from the leading group around 35 kilometres (22 mi) into the race.

[25] Hug broke away from American Daniel Romanchuk after 13 miles (21 km) of the race, and maintained it to the end,[25] eventually winning by over three minutes.

[5] In order to reduce crowding on the course, the event used a staggered start of competitors,[4] with 40 separate groups beginning over the course of 90 minutes.

[29] In July 2021, the race organisers were criticised for refusing to allow a pregnant woman to defer her entry to the mass participation event until 2022.

[35][36] Celebrity competitors included former England cricket captain Andrew Strauss, who ran to support his wife who died in 2018, former Health Secretary Matt Hancock, television presenters Chris Evans and Sophie Raworth,[36] former Olympic rower James Cracknell and EastEnders actress Tanya Franks.