Robert Downey Jr. had to turn down the role of Poseidon in Die Odyssee (2026) due to scheduling conflicts with this film.
The film was originally titled "Avengers: The Kang Dynasty" and was to feature Kang the Conqueror, a time-traveller villain from Ant-Man and the Wasp: Quantumania (2023) and who had gotten teased in Loki (2021). However, in 2023 Kang's actor Jonathan Majors was fired after facing assault charges brought by his former girlfriend. In 2024 Marvel replaced Kang with Doctor Victor von Doom as the film's antagonist and renamed the film to "Avengers: Doomsday".
Loki (2021) writer Jeff Loveness was initially hired to write the screenplay for the film in 2023. However after the Ant-Man sequel underperformed at the box-office, Loveness was replaced with Michael Waldron. Both had been picked to work on the Multiverse Saga due to their work on the sci-fi comedy Rick and Morty (2013), which featured protagonist Rick Sanchez travelling across space and time.
Robert Downey Jr. revealed on The Hollywood Reporter's 'Awards Chatter' podcast that he, Kevin Feige & the Russo Brothers agreed to return during the dual writers & actors strikes during 2023.
Destin Daniel Cretton was previously selected as director in 2022, when the film was planned to be "The Kang Dynasty", but stepped down in early 2024.