Original set for release on the 13 April 2018, in late August 2020, Williams co-starred in the repeatedly delayed Disney/Fox superhero horror film "The New Mutants". "The New York Times" said Williams portrayed the Marvel superhero Rahne Sinclair / Wolfsbane—a Scottish mutant who can turn into a wolf, but struggles to reconcile this with her religious beliefs -- "with endearing sincerity". Though the film received mixed to negative reviews her performance (along with Blu Hunt and Anya Taylor-Joy) was regarded as good by "The Hollywood Reporter", including adding "layers of panache and emotion" to her character. The film was released in "in theatres" which were partly open, with reduced capacity, due to the worldwide COVID-19 pandemic, in August 2020 (despite many other major Disney films being further delayed). At the July 2020 virtual San Diego Comic-Con Comic-Con@Home fan convention, a panel featuring Williams, with co-stars Taylor-Joy, Hunt, as well as Charlie Heaton, Alice Braga, Henry Zaga, plus writer-director Josh Boone and the original comic book artist Bill Sienkiewicz. The online panel opened with a "cheeky teaser trailer" that went through the film's five previous release dates since 13 April 2018. During the panel, Williams spoke about the same-sex romance between her character Rahne and Dani Moonstar (played by Hunt), that is at the heart of movie "It was wonderful to see a relationship like this in the typically masculine world of superheroes. It was lovely to see these two fragile women who protect one another and bring light out in each other." The "Los Angeles Times" describes Dani and Rahne's romantic relationship as feeling honest and a central part of the story, grounding the film with "a sense of humanity", while making the "New Mutants" both a rare LGBTQ inclusive superhero film, as well as groundbreaking for a Disney release.