Return to Hindi cinema and progression (2018–2022)
Roy made her full-fledged Hindi film debut alongside Akshay Kumar as a housewife in Reema Kagti's 2018 period sports film Gold, an Excel Entertainment production revolving around India's struggle for recognition in the Hockey Olympics. Jyoti Sharma Bawa of Hindustan Times noted, "Roy as Akshay's wife Monobina is just the right mixture of crotchety and charming. With her grasp of Bengali, she brings that extra something to the film's milieu." Devesh Sharma of Filmfare stated: "Mouni infuses the film with a dash of sensuality and her stern wife's role adds a bit of a comic touch as well." With global earnings of ₹1.5 billion, Gold emerged as a commercial success. Roy followed the success of Gold with an item number called "Gali Gali" in the Hindi version of K.G.F: Chapter 1, presented by Gold producers Ritesh Sidhwani and Farhan Akhtar.
In 2019, Roy made a cameo appearance in the final few episodes of Naagin 3 as Mahanaagrani Shivangi. Roy's first release in 2019 was Romeo Akbar Walter alongside John Abraham and Jackie Shroff. For her performance as a RAW agent, Bollywood Hungama wrote, "Mouni Roy has an interesting part to essay. Though she does well, her character gets a raw deal." Romeo Akbar Walter underperformed at the box office. In her next film the same year, Made in China, Roy played a disturbed housewife alongside Rajkummar Rao. Vibha Maru noted: "Mouni is a surprise package. Though given a small role, she comes with much power, owning every bit of the screen time she gets." Made in China proved to be a modest commercial success.
Roy played a pregnant RAW agent in her first OTT feature film, London Confidential. Joginder Tuteja from Rediff.com stated, "Mouni Roy shows conviction in playing her part and is in a different avatar from her earlier performances." In 2021, she played an actress in Velle alongside Abhay Deol. Hiren Kotwani of Times of India wrote, "Mouni has limited scope to perform. Nonetheless, she lights up the screen every time she appears in the film." Roy appeared as the judge in Dance India Dance Li'l Masters 5 in 2022.
The same year, Roy played the main antagonist Junoon in Brahmāstra: Part One – Shiva. Roy received rave reviews from critics and audience alike. Tushar Joshi of India Today noted, "Roy as the villain is terrific and breaks the Naagin stigma with this career-altering performance." Prateek Sur from News18 wrote, "The real find of the film, Roy is simply terrific and really holds herself in this elaborate cast and effortlessly shines as the nemesis." The film became the highest-grossing Hindi film of 2022. Her performance in the film earned her the IIFA Award for Best Supporting Actress and a nomination for the Filmfare Award for Best Supporting Actress.
