In the small-scale feature, "Write When You Get Work" (2018), Mortimer played a member of the elite class. A family drama, about a man with Alzheimer's, "Head Full of Honey" (2018), was Mortimer's next release. She played the wife of Matt Dillon's character. The "Los Angeles Times" gave the film a negative review, and thought Mortimer and the cast were "all over the place". She then joined the cast of "Mary Poppins Returns", a 2018 musical fantasy directed by Rob Marshall. It is loosely based on the book series Mary Poppins by P. L. Travers. The film was a box office hit, and Christopher Orr of "The Atlantic" magazine praised Mortimer's "charming" performance.