When his father was killed at the Battle of Fort Donelson in 1862, Reilly was sent by his mother to live with some of her relatives in Illinois, and he spent the next several years there.

Reilly showed early ability as a catcher, but by 1877, having become a professional artist with Strobridge, he began playing in the infield to avoid hand injuries that might threaten his lithography career.

His career highs in a season were 135 games played, 553 at bats, 112 runs, 170 hits, 35 doubles, 26 triples, 13 home runs, 103 RBI, 82 stolen bases, 34 walks, a .339 average, a .366 on-base percentage, a .551 slugging percentage, and 264 total bases.

He was also the first player to hit for the cycle on three occasions during his career, doing so twice in the American Association and once in the National League.

Ren Mulford, a writer for Sporting Life, reported in 1914 that Reilly was "a rare old bachelor devoted to his art, and seldom goes to the ball yard where he once reigned a ruling favorite.