McDowall was born on October 4, 1984, in Liverpool, England. She developed an affinity for drama while attending Wade Deacon High School, in Widnes, Cheshire. After high school, McDowall continued her acting studies at Laine Theatre Arts college in Epsom.
McDowall made her film debut as Lisa in the 2008 film Mamma Mia! The Movie, which was based on the stage musical of the same name. As of 2017, McDowall has five film and three television appearances, as well as voicing a character in the 2013 video game release Ryse: Son of Rome.
McDowall also has a background in theatre. She has played in Mel Brooks' hit musical "The Producers" in West End of London, playing the part of Swedish stunner "Ulla". Most recently she was starring in "Chicago", in which she played the lead role of Velma Kelly when the show was at the Cambridge Theatre, London, returning to the role from January 2012 at the Garrick Theatre.
McDowall married former The Bachelorette 2007 contestant Constantine Tzortzis on May 16, 2014 in Cumming, GA. Tzortzis had left The Bachlorette late in his season, admitting that he was not in love with the lead lady.