Mell was educated in Chicago, at St John Berchman’s Elementary School and St Scholastica High School. She then attended Cornell College with a dual major in political science and history, before earning a culinary arts degree from California Culinary Academy. She returned to Chicago in 2000 and began working at Christy Webber Landscape, Chicago’s largest landscaping company.
Mell is the daughter of long-time Alderman Richard Mell and the sister-in-law of Governor Rod Blagojevich. She is openly lesbian and attracted considerable media attention when she was arrested in 2004 while protesting her inability to get a same-sex marriage licence from the Cook County clerk's office. Her partner is Christin Baker. Her campaign has won the support of the Gay & Lesbian Victory Fund.
Mell currently serves on Mayor Richard M. Daley's Advisory Council for Human Relations and is active in Equality Illinois. She has received a National Organization for Women award for her activism, as well as the Howard Brown Cornerstone Award for community excellence.