At C.W. Post College, Postel had a brilliant lacrosse career, playing varsity from 1963-1965 and leading the Pioneers to the Lydecker Championship during those years. As an attackman for the Pioneers, Postel led the nation in scoring in 1963. In 1964, he earned honorable mention All-American honors for the attack position and was second in the nation in scoring. In 1965, Postel switched to the midfield and again led the nation in scoring, earning third team All-America honors as a midfielder. Postel captained the team in 1964 and 1965 and was selected as C.W. Post's team MVP in both seasons.

Postel began playing for the Long Island Lacrosse Club in 1966, served in Vietnam from 1967-69, then returned home to continue playing for Long Island until 1985. He was the team captain for eight years and chosen club MVP three times. He was selected to the U.S. Club All-Star team 11 times and received the national Club Advocate MVP Award in 1966 and 1970.

Postel played for the U.S. Team in the 1974 and 1978 Lacrosse World Championships, and was one of three team captains in 1978. He also has the unusual distinction of being selected for two positions on the U.S. Team - in 1974 as a midfielder and in 1978 as an attackman. He was the assistant coach for the U.S. Team at the 1986 World Championships.

Postel began his coaching career at Manhasset High School in 1969. He was honored as Nassau County's High School Coach of the Year in 1973 and 1974. He left Manhasset in 1975 to become the head coach at Suffolk Community College for five years. In 1978, he was selected National Junior College Coach of the Year. In 1986, he became the head coach at his alma mater, C.W. Post College.