Michael Joyce (Mike) O'Neill (September 7, 1877 - August 12, 1959) was a starting pitcher and left fielder in Major League Baseball. From 1901 through 1907, he played for the St. Louis Cardinals (1901-1904) and Cincinnati Reds (1907). O'Neill batted and threw right-handed. A native of Maam, Ireland, he played as Michael Joyce in his 1901 rookie year with the Cardinals.
O'Neill was a good-hitting pitcher who occasionally played in the left field. In 1901, he ended with a 2-2 record and a 1.32 earned run average, including a shutout, and hit .400 (6-for-15). His most productive season came in 1902, when he posted a 18-12 record with two shutouts, a 2.75 ERA, and two saves. On June 3, he was rested until being summoned as a pinch-hitter in the ninth inning with the bases loaded. O'Neill responded by hitting the first pinch grand slam in major league history off Togie Pittinger of the Boston Beaneaters. It was an inside-the-park home run as O'Neill also became the first National League pitcher to hit a grand slam in the 20th century.
Despite his 3.26 ERA in 1903, O'Neill had a disappointing 4-13 record, in part for a poorly run support, as he posted a WHIP of 1.56. He went 10-14 with a 2.09 ERA in 1904 and did not return with St. Louis the next year. He also played with the Cincinnati Reds in 1907, strictly as a reserve left fielder and pinch-hitter, retiring from baseball at the end of the season.