In 1936, Gallagher was signed by the New York Yankees and assigned to their affiliate in the Piedmont League, the Norfolk Tars. Gallagher performed well for the team, which was managed by former major leaguer first baseman Johnny Neun. That season, Gallagher played in 142 games, and finished the season with 19 home runs and a .348 batting average. Gallagher began to climb his way up the ladder in the Yankees farm system, eventually playing for their Double-A ball club, the Kansas City Blues. At Kansas City, Gallagher continued to show his power at the plate, leading the team with 24 home runs. He hit two more than teammate Wally Judnich.
In 1939, Gallagher made his debut in the major leagues, which made him briefly a teammate of Lou Gehrig. Gallagher was back-up to outfielder George Selkirk and made his MLB debut April 20, going 0–3 in the Yankees 2–0 win over the Boston Red Sox. On June 13 of his rookie season, the Yankees traded Gallagher to the St. Louis Browns for reserve infielder Roy Hughes. While with St. Louis, Gallagher batted .282 and hit nine home runs while appearing in 71 games. The Browns finished the season in last place.
On May 27, the Browns traded Gallagher to the Brooklyn Dodgers in exchange for Roy Cullenbine, and infielder/outfield. In 1946, at the age 32, Gallagher appeared in six games for the Montreal Royals, where he was teammates with Jackie Robinson. After his brief return, Gallagher retired from baseball.
In March 1947, he was hired as the head coach of the Stephen F. Austin Lumberjacks baseball team.[3] He held that job through the 1949 season, after which he resigned to attend the University of Houston and was replaced by Paul Geisler.[5] He was named the head coach of the Rice Owls baseball team prior to the 1962 season.[6]
National champion
Postseason invitational champion
Conference regular season champion
Conference regular season and conference tournament champion
Division regular season champion
Division regular season and conference tournament champion
Conference tournament champion