The confirmed appearance
ComicBook's July 2, 2025 interview with Ironheart creator Chinaka Hodge and director Angela Barnes identifies Sacha Baron Cohen as Mephisto in episode six, The Past Is the Past. Their discussion describes the casting choice for the series' finale, resolving the earlier speculation about which Marvel character Cohen would portray.
Source [1]Why this is more specific than an MCU rumor
The named episode and the filmmakers' interview are evidence for a particular performance. They do not establish an appearance in every later film associated with the character, or confirm fan theories about earlier shows. Comics history, casting rumors and a completed screen appearance are three different sources of claims.
The Eros guide linked below makes a similar distinction for Harry Styles's Eternals appearance. Both accounts keep the project and character explicit, helping readers follow the story without treating an old rumor as a new studio announcement or an open-ended contract.
Source [1]