McNeill won his first major title in 1938 when he defeated Frank Bowden at the U.S. National Indoor Tennis Championships, played at the Seventh Regiment Armory in Manhattan, New York.

[1] In 1939, McNeill became the second American to win the French Championships singles title (after Don Budge) when he defeated compatriot Bobby Riggs in the final in straight sets.

[4] In June 1940, McNeill beat Bobby Riggs to win the singles title at the U.S. Men's Clay Court Championships in Chicago.

[9] McNeill was the third player who managed to overcome a two-set deficit in the final of the U.S. Championships after Maurice McLoughlin (1912) and Bill Tilden (1922).

While stationed there he won the Argentinian Championships in 1942 and defended the title successfully in November 1943, defeating Pancho Segura in the final.