Mella Carroll was born in Dublin, her parents were Patrick Carroll (Commissioner of the Garda Síochána from May 1967 until his retirement in September 1968) and Agnes Mary Caulfield. Carroll attended Sacred Heart Convent School of Lower Leeson Street and then University College, Dublin where she graduated in French and German. Then she studied at King's Inns and was called to the Bar in 1957, and in 1976 was called to the Bar of Northern Ireland. In 1977 she became a Senior Counsel in the Republic of Ireland. She became the first woman to serve on the High Court of Ireland, called on 6 October 1980.
During her High Court career she delivered a number of important decisions. For instance the attempted ban on One Girl's War: Personal Exploits in MI5's Most Secret Station (the memoirs of Joan Miller) was declined by her after request by the Attorney General of England and Wales. She also delivered judgements in controversial cases on abortion, bin charging and unmarried mothers. She sat in the Central Criminal Court over the Catherine Nevin murder trial and subsequent retrial because of the jury being overheard in its deliberation. She retired from the courts in November 2005 due to long running illness she would have been entitled to serve until the following year.
Carroll chaired a number of high-profile commissions in the Republic; the County and County Borough Electoral Area Boundaries Commission (1984), the Commission on the Status of Women (1991), the Commission on Nursing (1997).
Carroll also held judicial positions in the administrative tribunal of the International Labour Organisation, Geneva, for a time being the vice-president. She was appointed Chancellor of Dublin City University (and Chair of the Governing Authority) in 2001 and held this post until her death. She never married.