He is the only amateur player to have won two US Open squash titles (in 1955 and 1959).
[1][3] He was runner-up at the US national doubles championship on nine further occasions (three of those times with his son Gil – his sons Gil and Drew won five national titles in all, including one together in 1986).
He was a Haverford College tennis team leader and member of a graduating class that accumulated four successive Middle Atlantic League championships and a four-year record of 45–6 while at Haverford.
[3] 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).