Barnston first appeared on television in the minor role of Daisy in the 2003 romantic drama The Young Visiters. The following year she played the character of Hester Merton in the BBC's five-part mystery drama, Shadowplay. The 2004 production about children in Victorian life was nominated for a BAFTA children's award for Best School Drama In 2006, Barnston starred in the CITV children's drama spin-off The New Worst Witch. The spin-off was cancelled after the second series had finished airing. In May 2008, Barnston appeared in two episodes of The Inbetweeners, playing Susie. Barnston has most recently played the role of Maya Kemp in BBC drama series Casualty.