Johnson attended Highgate Wood School and Performing Arts College. She is educated to A Level standard, but had originally planned to continue her education at a drama school. She was first spotted by an agent in a student production of Guys and Dolls in which she played the lead.
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 an acclaimed National Theatre production of Dylan Thomas' Under Milk Wood directed by Roger Michell and in Puccini's La bohème at the Royal Albert Hall.
Johnson left Eastenders in 2004 after two years and 194 episodes, and has since returned to stage acting. In 2005, Johnson played Juliet in an open air run of Romeo and Juliet at Stafford Castle opposite fellow soap opera alumni Bruno Langley, appeared as the lead in the acclaimed UK tour of Daisy Miller directed by Christopher Morahan and in the pantomime Cinderella at the Stafford Gatehouse.
Johnson appears in the 2008 film Adulthood, the sequel to the 2006 motion picture Kidulthood.
She has also made television appearances as a contestant on Strictly Come Dancing spin off Strictly Ice Dancing and the quiz show Test the Nation.