[1] Myles is a theatre actress, winning an Ian Charleson Award in 2004 for her performances in Royal Shakespeare Company productions of the plays Titus Andronicus and The Taming of the Shrew.

From 2003, Myles based herself in Stratford-upon-Avon, playing Lavinia in the Royal Shakespeare Company production of Titus Andronicus and Bianca in The Taming of The Shrew,[7] for both of which she received the Sunday Times Ian Charleson award in 2004.

"[11] Gwen, an audience surrogate character,[12] is characterised by Myles as Torchwood's "social worker", who "can run and fight and stand in her own corner and win."

[14] Myles also appeared in the first episode of Merlin, "The Dragon's Call", where she played Lady Helen of Mora and Mary Collins, a witch who impersonated her.

[17] 2009 saw her star in the independent Welsh film A Bit of Tom Jones, as well as the one-off drama Framed for BBC Wales, alongside Trevor Eve.

In July 2010, Myles announced she would appear in an original BBC Cymru Wales TV series, Baker Boys, co-written by Helen Raynor and Gary Owen and airing in early 2011.

[18][19] 2011 also saw Myles provide the voice of the Dalish elf Merrill in the Bioware video game Dragon Age II.

[1] In 2017, Myles starred alongside her real life husband Bradley Freegard in the Welsh drama Un Bore Mercher for S4C.

Myles played Faith Howells, a solicitor whose husband Evan, with whom she works at their family-run law firm, disappears whilst she is on maternity leave following the birth of their third child.

In 2018, Myles played the role of Norman Scott's lover Gwen Parry-Jones in the BBC drama A Very English Scandal.

The drama relates the real-life story of politician Jeremy Thorpe's affair with Scott and subsequent trial for attempted murder, based on the novel of the same name by John Preston.

In 2002,[27] 2003,[28] and 2009,[29] Myles was nominated for Best Actress in the BAFTA Cymru Awards for her role as Ceri on the BBC Wales drama Belonging.

[34] For her role as Gwen Cooper in Torchwood: Miracle Day, Myles was nominated for a Satellite Awards in the Best Television Actress category.

[50] Myles has described California as the "land of dreams", and her experience filming the fourth series of Torchwood in Hollywood as the best year of her life, adding that she would love to spend more time there.