In 2008, Fennell was commissioned to write a film script (co-produced by Madeleine Lloyd Webber). Called Chukka, it's a romantic comedy about a group of teenagers who fight the closure of their school by taking on the rich kids at polo.
She starred in the Channel 4 sitcom Chickens with Simon Bird, Joe Thomas and Jonny Sweet. Then she joined the cast of the BBC One series Call the Midwife as Patsy Mount, after dying her blonde hair red. She is also known for her film roles in Anna Karenina (2012) and Albert Nobbs (2011).
Her first novel was published by Bloomsbury Children's Books in January 2013: Shiverton Hall, a children's fantasy. In December 2012, it was released as an ebook by Bloomsbury USA.
The Creeper, a sequel, was published mid-2014. ISFDB catalogues them as the Shiverton Hall series. It was shortlisted for the Waterstones Children's Book Prize in 2014. She then released Monsters in September 2015, her first adult horror book.