"Zina and Patrick each possess that rare combination of heart, character and competitive fire that define leadership and are synonymous with the Olympic ideal," USTA President Alan Schwartz said.
The Olympics take place Aug. 13-28 in Athens.
McEnroe, 37, has been the U.S. Davis Cup captain since 2001, leading the team to a semifinal finish in 2002. He won one singles title and 16 doubles crowns during his nine years on the ATP Tour.
Garrison, 40, was named Federation Cup captain in December after serving as a coach since 1999. She was a member of the 1988 and 1992 U.S. Olympic teams, winning a doubles gold medal with Pam Shriver in 1988.