After his college career, he signed a contract with the Philadelphia Athletics of the American League, and joined the team for the 1932 season without having played in minor league baseball. He made his Major League Baseball debut on July 21, 1932, in the ninth inning of a game versus the St. Louis Browns. In one inning, he surrendered one run and did not receive a decision in the 5–3 loss. In his next appearance, on July 28, he started and pitched 2 innings in a loss to the Detroit Tigers. After making a short relief appearance on August 19, he started his second game of the season on September 24 against the Washington Senators. He pitched the first nine innings of the game and the score was tied 7-7. His relief then allowed a run in the 10th inning, and the Athletics lost. McKeithan began the 1933 baseball season with the Montreal Royals, a class-AA team in the International League. In 45 appearances for the Royals, he had an 8–6 win–loss record , and a 4.80 earned run average in 148 innings pitched. He then re-joined the Athletics in September, appearing in three additional games, including four innings of relief on September 18. He received the victory against the Tigers, allowed one run and struck out three batters, and connected for a base hit in his only at bat. His three strikeouts were the only three of his MLB pitching career. He returned to the Athletics for the 1934 season, and made his first appearance on April 18. He finished the game with two innings of relief and surrendered an eighth-inning home run to Babe Ruth, the first that he had allowed in his career. It is claimed that this was the longest home run hit by Ruth in his career. In May, he made two one-inning relief appearances, and allowed two runs to score in each, raising his season's ERA to 15.75. On May 22, the Athletics released him from the team. His final MLB totals include 25 innings pitched in 10 games, and a 7.36 ERA. McKeithan finished out the 1934 season with the Syracuse Chiefs of the IL, the class-AA affiliate of the Boston Red Sox. His win-loss record was 3-7 for the Chiefs in 22 appearances, and had a 6.21 ERA in 87 innings pitched. The following season, his last as a professional, he played for the Galveston Buccaneers of the Texas League, which was a class-A league. In eight games, his W–L record was 2–3 in 20 innings pitched.
