Walsh holds the record for lowest career earned run average, 1.82.

Walsh started his professional baseball career with the 1902 Meriden Silverites of the Connecticut State League.

[2] After playing the 1903 season with the Meriden Silverites and Newark Sailors of the Eastern League, the Chicago White Sox purchased Walsh's contract for $750 ($26,247 in today's dollars).

[3][4] Walsh made his major league debut in 1904 with the Chicago White Sox and pitched his first full season in 1906, going 17–13 with a 1.88 ERA and 171 strikeouts.

He also struck out at least one batter each inning of that game; this feat has since been duplicated only once, by Bob Gibson in the 1968 World Series opener.

An apocryphal story goes that architect Zachary Taylor Davis consulted Walsh in setting the park's field dimensions.

[10] Walsh inherited 24 runners in 1912 and allowed none of them to score, setting a Major League record that stands As of 2023[update].

"I could feel the muscles grind and wrench during the game, and it seemed to me my arm would leap out of my socket when I shot the ball across the plate", Walsh later recalled.

[12] He attempted a comeback with the Boston Braves in 1917, but was let go, ending his major league career.

[10] He has the third-lowest career WHIP in MLB history (1.00) and the lowest ever for someone with 10 or more seasons pitched.