George Diehl Mateer Jr. (June 4, 1928  – September 22, 2012) was an American hardball squash player and tennis player. He was one of the leading squash players in the United States in the 1950s and 1960s. He is the only amateur player to have won two US Open squash titles (in 1955 and 1959). He also won three U.S. National Singles titles between 1954 and 1960 and a record eleven U.S. National Doubles titles between 1949 and 1966. 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). Diehl also won two intercollegiate titles. He did not compete in the two other years that the intercollegiates were held due to a conflict (it being the same weekend as the US National Doubles, that he won that year).
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. He is also a member of the US Intercollegiate Hall of Fame, the Episcopal Academy Hall of Fame, the Maryland Squash Hall of Fame and the Haverford College Glasser Hall of Achievement.
As a younger boy, Mateer was noted for his tennis. 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. 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).
- "Hall of Fame: G. Diehl Mateer, Jr.". Philadelphia Squash Racquets Association. Retrieved March 2, 2010.
- "G. Diehl Mateer Jr.". Main Line Media News. 26 September 2012. Retrieved 12 November 2014.
- "G. Diehl Mateer, Jr. Class of 1950 - Squash, Tennis". Haverford College. Retrieved March 2, 2010.
- "Historical Information about the US and North American Opens.". Retrieved March 2, 2010.
- "Individuals (M)". College Squash Association. Retrieved March 2, 2010.
- "Hall of Fame (M)". College Squash Association. Retrieved March 2, 2010.
- "The Episcopal Academy Alumni Society Athletic Hall of Fame" (PDF). Retrieved March 2, 2010.[dead link]
- "Athletics Alumni - Careers". Haverford College. Retrieved March 2, 2010.