At the age of 17, Cummings made his debut in the National Association of Base Ball Players with the Excelsior senior team.

For his performance, he received prophetic praise from the Brooklyn Eagle: "[H]e has only to keep on in the way he has begun, and he will one day (not far distant) be ranked among the best pitchers of the country.

[3][6] After the latter game, baseball writer Henry Chadwick commented on the skills of the young Cummings and his promising future with the Excelsior club.

[8][9] Cummings, who stood 5'9" and weighed 120 pounds,[10] compiled a 145–94 career record and 2.42 earned run average while playing for five different professional teams from 1872 to 1877.

Between 1872 and 1875, he pitched in the National Association of Professional Base Ball Players (NA) with the New York Mutuals, Baltimore Canaries, Philadelphia White Stockings and Hartford Dark Blues.

[13][3] In the middle of the 1877, he left the Live Oaks and returned to the NA, pitching for the Cincinnati Reds, where he appeared in only 19 games and posted a 5–14 win-loss record.

[14][10] Among other records, Cummings was the first player to pitch two complete games in one day: September 9, 1876, when he beat the Cincinnati Red Stockings twice, 14–4 and 8–4.

[18][19] Cummings first used the curveball in competition while pitching for Brooklyn's Excelsior club, in a game on October 7, 1867, against the Harvard College team.

[24] According to a 2002 article by ESPN's Steve Wulf, Cummings was "fairly well-connected" in baseball, as evidenced by his position with the International Association.

Baseball leaders Chadwick, Harry Wright and Albert Spalding, among others, credited Cummings as the inventor of the curveball.