Harker has acted in both contemporary and classic works, on stage, in movies and in TV series. In 1990–91 she appeared alongside Clive Owen in Chancer. She later played Dinah Morris in the 1991 adaptation of Adam Bede. She starred as Jane Bennet in the 1995 TV adaptation of Jane Austen's Pride and Prejudice. She is featured as Emma Fitzgerald, the love interest of Superintendent Tyburn (Trevor Eve) in the BBC TV series, Heat of the Sun (1998). She appeared in the play, On the Shore of the Wide World.
She was married to Iain Glen from 1993 to 2004; they have one son, Finlay (born 1994). She was later in a relationship with Paul McGann from 2006 to 2008.
She played Sapphire in Big Finish Productions' audio revival of Sapphire & Steel, in three series of plays released on CD between 2005 and 2008. In 2003 she played Clare Keightley in the audio version of the Doctor Who adventure Shada, alongside Paul McGann. In December 2011 Harker appeared in the BBC drama Young James Herriot.
In 2012 she returned to the stage, playing the role of Sue in a London production of Mike Leigh's Abigail's Party.
During early 2014 she starred in the Gate Theatre, Dublin production of The Vortex by Noël Coward. In 2015, she played Miss Ella Rentheim in a BBC Radio 4 production of Ibsen's John Gabriel Borkman.
