2018–present: To All the Boys franchise and XO, Kitty
Cathcart joined the cast of the film adaption of Jenny Han's bestselling book To All the Boys I've Loved Before, in which she was to portray 11-year-old Kitty Song-Covey, the younger sister of protagonist Lara Jean. The film was released in 2018 on Netflix and quickly became one of the most viewed Netflix original films. The film received generally favourable reviews and critics praised the actors. Molly Freeman of Screen Rant called Cathcart a "hilarious standout" while Charlie Salek of Joker Mag called the supporting performances "crucial".
In November 2018, Cathcart was cast in the Brat web-based series Zoe Valentine in which she was to portray the eponymous character. Cathcart had not auditioned but instead was contacted about the role. The first episode, which premiered on January 17, 2019, received 4.9 million views on YouTube as of November 2020. Cathcart described filming as being "crazy", saying "[it was] such a blur but so much fun". Cathcart starred as Zoe Valentine again in Brat film Spring Breakaway which was released on March 15, 2019, on YouTube. Zoe Valentine was later renewed for a second season. Cathcart reprised her role as Dizzy Tremaine in the third instalment of the Descendants franchise, Descendants 3 in 2019.
Cathcart reprised her role as Kitty in 2020 in To All the Boys: P.S. I Still Love You which had been filmed from March to May 2019, the film being primarily filmed in Vancouver. Released in February 2020, the film received generally mixed reviews. Cathcart received praise for her performance, with The Post writing that Cathcart was one of the only members of the returning cast to bring the "same type of energy as [she] did in the first film." She was nominated for the Cogeco Fund Audience Choice Award.. In mid-2020, because of the restrictions placed due to the COVID-19 pandemic, Cathcart hosted a virtual series called Letters To which allowed fans to connect with their favourite authors. Developed by Picturestart and released on Facebook Watch, Glitter Magazine wrote that the series had "all literary fans buzzing".
Cathcart reprised her role in the third film of the To All the Boys I've Loved Before film series titled To All the Boys: Always and Forever, Lara Jean, which was mostly filmed in Vancouver, New York and Seoul from July to September 2019.
Cathcart played Molly in the 2021 Disney Channel Original Movie Spin. Principal photography for Spin began in early October 2020 and wrapped in late November. Cathcart's performance was commended by Amy Amatangelo of Paste, who called her the "generation's secret weapon" and stated that projects including her were certain to be of high quality. She also reprised her role as Dizzy in the animated special Descendants: The Royal Wedding, which aired after Spin.
In March 2021, it was announced Cathcart would reprise her role as Kitty Song-Covey in a To All the Boys spin-off series, later revealed to be titled XO, Kitty. TheWrap described her as "appropriately sweet", while Comic Book Resources stated she "makes a confident jump to a leading role".
