The role of Vicki Fowler on the BBC soap opera EastEnders was Johnson's first television role though not her professional début, having previously appeared in numerous stage productions, including a six-month run in a National Theatre production of Dylan Thomas' Under Milk Wood directed by Roger Michell. Johnson left EastEnders in 2004 after two years and 194 episodes, and returned to stage acting as well as embarking on career in films. For her portrayal of Vicki Fowler, Johnson was voted #83 in 'The 100 Greatest Eastender's Characters Of All Time'.
In 2005, she played Juliet in an open air run of Romeo and Juliet at Stafford Castle and received favourable notices including in The Stage. In the same year she also appeared as the lead in the acclaimed UK tour of Henry James' Daisy Miller directed by Christopher Morahan and in the pantomime Cinderella at Stafford's Gatehouse Theatre.
In 2009 she played Mindy in a revival of Aunt Dan and Lemon at the Royal Court Theatre receiving good notices in the Evening Standard, The Guardian, The Times etc. In the same year she played the role of Helen in the horror film The Reeds.
In April 2010, Johnson was cast by Warner Bros. in the CW network's US TV pilot Damn Thorpes (aka The Wyoming Project) opposite co-star Sean Faris (Vampire Diaries). In the same year she also played the lead role in the UK film Panic Button and in the Channel 4 comedy Pete Versus Life as Trish, the girlfriend of Pete's friend Ollie. In February–March 2010 she appeared in the play Slaves for the Olivier award-winning Theatre 503, London.
She played the role of Laura Derbyshire, a pregnant teenager and subsequently a new mother, in the BBC Three comedy Pramface alongside her Beaver Falls co-star Emer Kenny. The first series piloted in late February 2012 and was well received by critics. She reprised her role for the second series which began airing in the new year of 2013 and was broadcast for a second time on BBC One throughout the summer of 2013. She reprised her role again for a third and final series which started airing in late February 2014.
