She previously represented the riding of Sudbury East, which was eliminated when the Mike Harris government redrew the boundaries of the provincial ridings to match the federal boundaries. Sudbury East had been represented by Martel's father, Elie Martel, from its creation in 1961 until his retirement in 1987. Shelley Martel sought and won the NDP nomination vacated by her father, and won the election that year.
In 1990, she was named Minister of Mines and Northern Development in the cabinet of Bob Rae after the NDP won the general election. In 1991, she resigned from cabinet after a scandal in which she told a guest at a cocktail party that she had seen confidential information about an Ontario doctor's OHIP billing files, and later claimed she had been lying. Towards the end of Rae's mandate, she was brought back into cabinet in another portfolio. However, the controversy did not dim her electoral prospects, as the voters in Sudbury East returned her to Queen's Park in 1995, and when the riding of Sudbury East was eliminated for the 1999 election, she easily won the nomination in the amalgamated Nickel Belt riding, and was re-elected again in 1999 and 2003.
Martel is married to NDP leader Howard Hampton, with whom she has two children.