Harmer began her stage career at age six, when she appeared in a musical production of "Tommy" at the West End of London. In 1997, at age eight, she made her first British television appearance in a series called "Pie in the Sky." Her next success came when she appeared in a British stage production of "Peter Pan," as the main character. Harmer also was an extra in the movie, "Harry Potter and the Sorcerer's Stone."
Currently, Harmer has two successful television shows airing regularly on British television. In "The Story of Tracy Beaker," Harmer portrays a care worker, a type of social worker. Her other role, in the series "Dani's House," portrays her as a 17 year old who is raising her two younger male siblings. Both shows have been well received in the United Kingdom.
In addition to her stage, movie and television appearances, Harmer has released several musical recordings, with more being planned in the near future. Her biggest hit to date is the single, "Free," which also is the theme song for "Dani's House."