[1][2] In 1867 the 19-year-old McMullin was a regular outfielder for the Keystone club of Philadelphia in the nominally amateur National Association of Base Ball Players.

For the 1870 season at age 21, McMullin became a regular pitcher for the Haymakers of Troy, New York, a pro team of average strength, with a powerful lineup but not much pitching.

In the slugfest, the Troy pitcher gave up all 49 Philadelphia runs (31 of which were earned) on 42 hits and seven walks, while striking out none.

[4][5] The lone game McMullin did not complete took place on September 5 against the Rockford Forest Citys, when he gave up 15 runs (nine earned) in the first eight innings and switched positions with shortstop Dickie Flowers for the ninth.

[6] In total, McMullin pitched 249 innings in 1871 and compiled a 12–15 won–lost record, a 5.53 earned run average, and 12 strikeouts.

[7] McMullin made 3 appearances at pitcher during the year, the first being a start on April 24 against the Washington Nationals, in New York's third game of the season.

[1] His final appearance as a pitcher in 1872 came on June 8, on the other side of a lopsided game, as he entered with a 15-run deficit and pitched the last two innings of an eventual 19-0 loss to the Philadelphia Athletics.

He made one appearance at pitcher, a complete game victory in which he pitched eight innings and allowed five runs (two earned) on ten hits and a walk.

[14] After not pitching at all the prior year, McMullin made four relief appearances for the Whites, posting a 7.94 ERA in 11.1 innings.