Marshal was born in Manchester, England.

Marshal has performed in more than ten plays in London. She has also appeared in six films, including The Hours alongside Nicole Kidman, and has made several television appearances. In 2007 she reprised her role as Cleopatra for season two of HBO's Rome, in which she appeared in three episodes. Most recently she has played Lucy Jaggat in series 2 of the BBC Three drama Being Human.

Marshal has performed in more than ten plays in London. She has also appeared in six films, including The Hours alongside Nicole Kidman, and has made several television appearances. In 2007 she reprised her role as Cleopatra for season two of HBO's Rome, in which she appeared in three episodes. Most recently she has played Lucy Jaggat in series 2 of the BBC Three drama Being Human.