McGrillis had a role in
The Pitmen Painters, by
Lee Hall, in 2007. It transferred to London's
National Theatre and then to
Broadway. When
Eve Best, the star of the show, fell ill, director
Jeremy Herrin asked McGrillis to step up to play Beatrice. She then appeared in
The Pass with
Russell Tovey at the
Royal Court Theatre in 2014. After appearing in
Hebburn, McGrillis was the regular character DS Rachel Coles in
Inspector George Gently with
Martin Shaw. McGrillis starred opposite
Academy Award nominee
Lesley Manville in the
BBC sitcom Mum (2018) as Kelly, the girlfriend of Manville's son, Jason. In February 2019, she played Hannah Wilde in episode eight of the eighth season of
Death in Paradise. She later played a grass-roots Manchester politician in the third series of
Channel 4 Paul Abbott police drama
No Offence and, in 2020, appeared as 'Mad Kay' in BBC sitcom
King Gary, written by
Tom Davis and as Sandra in the Channel 4 drama
Deadwater Fell. On 17 January 2022, it was announced that McGrillis would star as Courtney in BBC comedy
Avoidance (2022), alongside show creator
Romesh Ranganathan and
Jessica Knappett. In 2023, she played the main role of Helen Cavendish in the
ITV drama
Maternal (2023). In 2024, she appeared alongside
David Tennant,
Aidan Turner,
Danny Dyer and
Katherine Parkinson in the
Disney+ adaptation of
Jilly Cooper's
Rivals (2024). McGrillis was cast as Adrian Mole’s mother, alongside
Oliver Savell as Mole, in a television adaptation of the Sue Townsend 1982 novel
The Secret Diary of Adrian Mole, Aged 13¾, with
David Nicholls and
Caitlin Moran among the writers and filming taking place in 2026. ==Personal life==