2018–present: Diversification and leading parts
In 2018, Wallis landed a lead in the American comedy Tag as Rebecca Crosby. Later that year, she reunited with film director Alex Kurtzman for the Star Trek: Short Treks episode "Calypso", where she voiced the artificial intelligence Zora. She later reprised the character in a recurring role on Star Trek: Discovery.
In April 2018, she was announced as Cartier's ambassador for jewellery and the new face of the Panthère de Cartier watch.
In 2019, Wallis joined Showtime's mini-series The Loudest Voice as Laurie Luhn, Head of Booking at Fox News Channel and Roger Ailes's long-time "mistress". Luhn is one of Ailes's first victims to break silence and denounce him, alongside Gretchen Carlson. The series received generally favourable reviews and was nominated at the Golden Globe Awards for Best Limited Series or Television Film. Wallis's performance was praised and seen as "heartbreakingly powerful work" by the Chicago Sun Times. Later that year, Wallis reprised her role as Grace Shelby in the fifth season of Peaky Blinders.
In 2020, she starred in Robin Pront's thriller film The Silencing. Its premiere at the South by Southwest festival in March 2020 was cancelled due to the COVID-19 pandemic; it was finally released on DirecTV on 16 July 2020.
Also in 2020, Wallis starred as Princess Buttercup in Home Movie: The Princess Bride for Quibi to raise money for World Central Kitchen and she joined the cast of Floria Sigismondi's third film, The Silence of Mercy. Shooting ended in March 2021.
In 2021, Wallis starred in James Wan's horror film Malignant. Scheduled by Warner Bros to premiere in August 2020, the release was postponed due to the COVID-19 pandemic, opening on 10 September 2021, simultaneously in theaters and on HBO Max. The film is a tribute to the giallo genre, and follows Madison Mitchell, played by Wallis, who is "paralyzed by shocking visions of grisly murders," and as her torment continues, "discovers that these waking dreams are in fact terrifying realities." It is the first R-rated US movie to be released in China.
That same year, Wallis appeared in the science fiction action film Boss Level directed by Joe Carnahan, starring Frank Grillo and Mel Gibson and starred in Agata Alexander's science fiction thriller Warning, alongside Alice Eve and Alex Pettyfer. She also appeared in the Christmas black comedy Silent Night, directed by Camille Griffin, premiered at the Toronto International Film Festival in September. Wallis stars alongside Keira Knightley, Matthew Goode and Roman Griffin Davis.
In March 2024, Wallis joined Amazon MGM Studios' sci-fi thriller entitled Mercy, alongside Rebecca Ferguson and Chris Pratt. She also starred in Renato De Maria's 2024 Netflix film, Vanished into the Night, alongside Riccardo Scamarcio.