Madeley grew up in London, attending North London Collegiate School, and began her career as a child actress. She then trained at the Central School of Speech and Drama.

Madeley has performed three seasons with the Royal Shakespeare Company: 2001–2002; and 2003–2004. She acted in The Roman Actor opposite Sir Antony Sher.

In 2005 she appeared in three acclaimed off-West End productions (Laura Wade's Colder Than Here, as well as The Philanthropist (directed by David Grindley) and The Cosmonaut's Last Message..., both at the Donmar Warehouse), and rounded off the year with a starring role in the National Theatre's adaptation of Jamilla Gavin's novel Coram Boy.

In 2006, Madeley starred in two BBC TV movies – firstly as the title character in The Secret Life of Mrs Beeton, as well as in the original drama Aftersun – and the high profile ITV drama The Outsiders.

In 2007, Madeley appeared in Channel 4's Consent, which combined a dramatised vignette about an alleged date rape with a "real life" sequence in which lawyers and a jury made up of members of the public participated in a trial. In February 2007, Madeley played Nina in a production of The Seagull for a time, when the main actress fell ill.

She was the only cast member to reprise her role in Grindley's 2009 Broadway production of The Philanthropist.

In 2010 she appeared The Secret Diaries of Miss Anne Lister, based on a script by Jane English, and starring Maxine Peake as Anne Lister, a nineteenth century industrialist who was Britain's "first modern lesbian" and who kept a detailed journal. The film will be the Opening Night film at the Frameline Film Festival at the Castro Theatre in San Francisco on June 17, 2010.