Johnson began attending drama classes aged eight and landed her first professional acting *** 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 ****. 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 *** 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 *** One in August 2013.