Monika Hellwig was born in 1929 to a Catholic father and a Jewish (catholic) mother. They lived in Breslau and came round 1938 to Venlo in Holland because of the nazi regime. Her mother was Marianne Hellwig-Blaauw an artist that studied and worked at the famous Kunst Akademie in Breslau. With the boat from Hoek van Holland, the children were sent to familie in Scotland. Marianne stayed in Holland and start working in 1939 in Brunssum at the KAB (Kleiatelier Brunssum) as a teacher in art. As an artist Marianne made religious art. When the war started in Holland the nazi's forbid the city of Brunssum to employ her. She never went abrought to be reunited with her familie. Meanwhile Monica and her Jewish grandparents, moved several times to avoid the Nazis. As already told atfirst, the whole family moved from Breslau to the Netherlands, where Hellwig's father was killed in a car accident. Later, to avoid the Nazis invading the Netherlands, the eight-year-old Hellwig and her sisters were sent by their mother to Scotland to her parents to a boarding school. She was briefly reunited with her mother in 1946, but her mother died soon after the reunion. That same year, at the age of 15, Hellwig began her higher education at the University of Liverpool, from which she received degrees in law (1949) and social science (1951). She briefly relocated from England to the United States thereafter, where she both attended Catholic University for her master's degree in theology (1956) and joined the Washington chapter of the Medical Mission Sisters, a Roman Catholic religious order for women. A few years into her career, she returned to the University for a doctoral degree in theology (1966).

As a nun, Monika Hellwig was a research assistant to a Vatican official during the Second Vatican Council, one of the few women allowed as an unfettered observer at Council sessions. Fourteen years after she took her vows, Hellwig's order released her to permit her other work.

Thereafter, in addition to lecturing at many universities, Hellwig taught for more than three decades at Georgetown University, including six years as the Landegger Distinguished Professor of Theology. She also wrote many books, including Understanding Catholicism (1981), Jesus, the Compassion of God (1992), and The Eucharist and the Hunger of the World (1976). As President of the Catholic Theological Society of America, Hellwig in 1986 signed a controversial letter in support of Father Charles E. Curran, a Catholic University of America (CUA) professor, who "had been stripped of his authority to teach in Catholic universities because of his dissent from the church's teachings on such issues as contraception and homosexuality." In 1996, became president and executive director of the Association of Catholic Colleges and Universities, retaining the position until a few months before her death. She was a senior research fellow at Georgetown's Woodstock Theological Center at the time of her death.

Hellwig adopted two sons and a daughter, who survived her. She retained her British nationality while working and living in the United States.

Hellwig died on September 30, 2005, aged 75, at Washington Hospital Center from a cerebral hemorrhage. Following her passing, the National Catholic Reporter referred to Hellwig's"[n]ear encyclopedic knowledge of Catholicism, which might be expected of the coauthor of the Modern Catholic Encyclopedia."