Johnson began attending drama classes aged eight and landed her first professional acting job at 11 in the film Dear Frankie, starring Emily Mortimer and Gerard Butler. From the age of 13, Johnson played Nicki Cullen in the Glasgow-set soap opera River City. She left in 2009, after six years, to train at the American Academy of Dramatic Arts in New York before returning to Scotland after being hand-picked for the lead role of Paddy Meehan in BBC Scotland's adaptation of The Field of Blood for which she won the 2011 BAFTA Scotland Award for Best Television Actor/Actress ahead of co-stars Peter Capaldi and Ford Kiernan. Johnson reprised the role of Paddy Meehan for The Field of Blood: The Dead Hour which began filming in Glasgow in November 2012 and was broadcast on BBC One in August 2013.