Gerritt Paul Groen Jr., known as Paul, was born in Chicago, Illinois on June 9, 1937. His family moved to Wheaton, Illinois, around 1947. Groen was educated at Wheaton Christian Grammar School and Wheaton Academy (1951-1955). He attended Wheaton College (1955-1959), majoring in pre-med and intending to be a medical missionary. While a student, he also participated in sports and was a member of an evangelistic team which presented both music and message. He received his medical training at Baylor College of Medicine (1959-1963), after which he completed his internship at Philadelphia General Hospital (1963-1964).

Groen served as a missionary doctor with the Christian Reformed Church among the Tiv people in Nigeria (1964 to 1973). While based in Mkar for most of his term of service, he also served temporarily in Takum, which was located in a Hausa area. In 1965, he married Maxine Ohlman, also a missionary in Nigeria. He concluded his missionary service in 1973 in order to expand his medical experience. Upon returning to the United States, Groen studied at the Henry Ford Hospital in Detroit, specializing in orthopedic medicine (1973-1976). He then returned to Wheaton, where he joined a medical practice (1976-1980). In 1980, he co-founded Sportsmed, a program combining a sports health club with orthopedic medical services in Wheaton. While living in Wheaton, he also served Wheaton College as one of its campus physicians. In 1992, Groen moved to Brunswick, Georgia, where he joined the staff of MAP International (formerly Medical Assistance Program) as Vice President of Worldwide Services. The Groens had two sons, G. Paul III and Joel.