Brealey has written on cinema, art and music since her teens, contributing reviews and features for magazines including Premiere UK, Empire, SKY, The Face, Neon and Total Film.

[5] Brealey made her TV debut as Nurse Roxanne Bird in two series of BBC drama Casualty before playing Judy Smallweed in Bleak House.

Brealey followed Bleak House with a comic turn as Anorak, Alistair MacGowan's black-bobbed sidekick, in comedy drama Mayo, described by The Hollywood Reporter as "Agatha Christie does Moonlighting".

[7] Brealey is often asked to work in accents, playing a doughty Yorkshire doctor in Ripper Street, a Cockney ne'er-do-well in Law & Order: UK, a broken Geordie widow in Inspector George Gently and a ball-breaking Edinburgh academic in Clique.

Her big disadvantage is that she’s too attractive for ‘plain’ Sonya, but she disguises this by suggesting a lack of sexual allure with awkward giggles, squirrelly movements and a stupefied beaming naivety.

All brilliantly done..." In 2011 Brealey was the sex-mad, short-frocked daughter of Julian Barratt and Doon Mackichan at the Young Vic in Richard Jones's Government Inspector.

Brealey is the narrator of Caitlin Moran's How to Build a Girl and its sequel How to Be Famous, Alex Michaelides’ The Silent Patient, Kate Mosse's Number One Bestseller Labyrinth and Hallie Rubenhold's The Five: The Untold Lives of the Women Killed by Jack the Ripper.