[2] Williamson began her career as an actress under the stage name Lisa Cassidy, appearing in a number of pantomimes and the 2014 John Lewis Christmas advert.
[3] This experience inspired Williamson's debut young adult novel The Art of Being Normal, told from the perspective of a transgender teenager.
[6][7] This was followed by Williamson's second novel All About Mia in 2017, which explores sibling dynamics,[8] and her third novel Paper Avalanche, which subverts the messy teenager stereotype with a hoarder mother and neat daughter, in 2019,[9] both published by David Fickling Books.
[11] In addition, Williamson collaborated on the 2018 novel Floored with Sara Barnard, Holly Bourne, Tanya Byrne, Non Pratt, and Eleanor Wood.
[15][16] In 2023, Williamson moved into middle grade fiction with the Bigg School series, acquired by Guppy Books, starting with Best Friends Forever.