Mella Carroll was born in Dublin, her parents were Patrick Carroll (founder member and 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. For a time, she was the only female Senior Counsel practising in the Irish State.
Carroll became the first woman to serve as a judge in the High Court of Ireland, called on 6 October 1980. She was addressed as 'my lord' by barristers in her court for 10 years until she announced she would prefer to be called 'judge'. 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 a 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 after 25 years, due to long-running illness.
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.