Mella entered political life, having been an unsuccessful candidate in the 1989 provincial election as a member of the Prince Edward Island Progressive Conservative Party. She served as the Leader of the party from 1990 to 1996 and Leader of the Opposition from 1993 to 1996. She represented 3rd Queens from 1993 to 1996, and then Glen Stewart-Bellevue Cove from 1996 to 2003.
Elected with the government of Premier Pat Binns in 1996, she served as Provincial Treasurer (Minister of Finance) until 2003.
In 2004, Mella was appointed with Douglas MacArthur and Matt Power to chair the Prince Edward Island Conservative Party of Canada federal election campaign.