Career
He was one of the leading squash players in the United States in the 1950s and 1960s. In tennis, Mateer reached the second round of the 1951 United States. National Championships and lost in the first round in 1948. Mateer was inducted into nearly every possible hall of fame, including the United States Squash Racquets Association Hall of Fame in 2000, the initial year.
As a younger boy, Mateer was noted for his tennis.
He coached the Haverford College squash club team in the late 1970s. Mateer married Joan Sommer and had five children: G. Diehl III, Andrew (Drew), Gilbert (Gil), Carver Severance, and Jeffrey (Jeff).