As a 19-year-old player, Van Horn reached the finals of the 1939 U.S. Championships beating John Bromwich[2] only to lose to Bobby Riggs in just 56 minutes (6–4, 6–2, 6–4).

Van Horn also won the United States Pro Championship in 1945.

[3] He lived briefly in Atlanta, Ga., where he had been hired as Head Tennis Professional at the Piedmont Driving Club.

In 1951, he moved to Puerto Rico as a coach at the Caribe Hilton Swim and Tennis Club, where he worked with many promising juniors, the most notable being Charlie Pasarell who was ranked No.1 in the U.S. in 1967 who he continued to coach on the main tour, another notable junior was Manuel Diaz, later to become a star on the University of Georgia tennis team and UGA coach.

His career as a coach spawned institutions such as the Welby Van Horn Tennis Academy in Boca Raton, Florida, and Welby Van Horn Tennis programs in a number of locations.