Merchant's early credits include Margo Durrell in the 2005 BBC television film My Family and Other Animals. In 2006 she appeared in two more BBC dramas, as Sara Fox, the daughter of Alan Davies and Michelle Gomez, in the comedy The Good Housekeeping Guide, and as Probationer Eastwood in the historical drama Casualty 1906.
In 2007, while studying at the University of Cambridge, she performed in an audio production of Lady Windermere's Fan as Lady Agatha, while in 2008 she appeared in the ensemble drama Radio Cape Cod in which she plays a summer theatre camper.
She was cast in the third season (2009) of The Tudors, playing Catherine Howard, the fifth wife of Henry VIII. She debuted in the part in the third-season finale and features heavily in several episodes of the fourth (and final) season.
She has also starred in DCI Banks: 'Playing with Fire' as Miranda. In early 2012 Merchant played the 17-year-old Rosa Bud in the BBC adaptation of Dickens' The Mystery Of Edwin Drood, written by Gwyneth Hughes (Five Days, U Be Dead, Miss Austen Regrets and Blood Strangers).
Merchant is a patron of the international development charity Build Africa. In March 2013 she gave a speech at an International Women's Day event at London's Hospital Club.
