Ferris was born in Hanover, Lower Saxony, Germany, to Welsh parents, while her father was serving in the Royal Air Force. Ferris spent her childhood in the Aberkenfig area, near Bridgend in Wales. Her father, Fred Ferris, was a policeman, and her mother, Ann Perkins, worked in her family's bakery business. Her family emigrated to New Zealand when she was 13. Ferris returned to the UK in her early 20s. In 1986, she married actor Roger Frost. She lives in Elham, Kent, South East England.
Ferris performed in her younger years at the Mercury Theatre in Auckland, and later with various regional companies in the UK.
Ferris played motherly Ma Larkin in the ITV series The Darling Buds of May, which ran from 1991 to 1993. She appeared in The World of Peter Rabbit and Friends as Aunt Pettitoes in The Tale of Pigling Bland in 1995. She has also acted in a succession of television dramas. These include: Meantime, in which she played the Cockney mother of Phil Daniels and Tim Roth, Connie, Hardwicke House, Oranges Are Not the Only Fruit, Where the Heart Is and Paradise Heights. From 2003 to 2006, she played the gardening sleuth, Laura Thyme, in Rosemary & Thyme.
Her roles in costume dramas include parts in television adaptations of Middlemarch, The Tenant of Wildfell Hall, Our Mutual Friend, The Turn of the Screw, Pollyanna and Jane Eyre.
In 1996, Ferris portrayed the brutish, authoritarian school headmistress Agatha Trunchbull in Matilda. Additionally, she played the callous Aunt Marge in Harry Potter and the Prisoner of Azkaban. Following her roles as those two unpleasant characters, Ferris took on the role of Miriam, a motherly activist in Children of Men; that film and the one before were both directed by Alfonso Cuarón. She appeared in Agatha Christie's Marple's 4.50 from Paddington in 2004 as Mrs Elspeth McGillicuddy.
She has also acted in productions for BBC Radio 4.
Her career in the theatre has included parts in Royal Court Theatre and National Theatre productions. In 2007 she played Phoebe Rice in an acclaimed revival of John Osborne's The Entertainer at London's Old Vic Theatre.
In 2007 Ferris took part in the BBC Wales programme Coming Home about her Welsh family history.
In 2008 she played Mrs General in an all-star cast BBC adaptation of the Charles Dickens novel Little Dorrit. From 2009 to 2010, she appeared in the final series of the BBC comedy Gavin & Stacey as Cath Smith. In 2010 she made a guest appearance in the sitcom Grandma's House.
From 2012 to 2016 Ferris played the part of Sister Evangelina in the series Call the Midwife.
In June and July 2015 Ferris was the guest of Sarah Walker on BBC Radio 3's Essential Classics.