McHugh was a visiting lecturer in theology at Princeton Theological Seminary (1974), Immaculate Conception Seminary (1976–81), and American College of Louvain in Belgium (1976). He became director of Diocesan Development Program for Natural Family Planning in 1981. From 1978 to 1981, he studied moral theology with a concentration in medical ethics at the Pontifical University of St. Thomas Aquinas (Angelicum) in Rome where he earned a doctorate in theology. He then served as a visiting lecturer at the Pontifical Lateran University in 1982. He served as special assistant at the World Synod of Bishops on "The Christian Family in the Contemporary World" in 1980, and was appointed to the delegation of the Permanent Observer Mission of the Holy See to the United Nations in 1983. He was appointed archdiocesan vicar for Parish and Family Life in 1986.