Maas was a member of the 1958 World Series champion Yankees. He saw action in one game, relieving Bob Turley during the first inning of Game 2 when the Milwaukee Braves scored seven runs.
After making his big-league debut in 1955 with Detroit and then struggling through an 0–7 campaign in 1956, Maas put together a 10-win season in 1957. In a midseason transaction the following June, he and fellow pitcher Virgil Trucks were traded to the Yankees for outfielder Harry "Suitcase" Simpson and pitcher Bob Grim.
Maas got seven wins for New York in the second half of that pennant-winning season, then went 14–8 for them in 1959. He also pitched two innings of relief for the Yankees in the 1960 World Series against the Pittsburgh Pirates.
He was chosen by the Los Angeles Angels in the American League expansion draft prior to the 1961 season, but never played for them in a regular season game before being traded back to the Yankees, with whom he concluded his career.
For his career, he compiled a 45–44 record with a 4.19 earned run average and 356 strikeouts in 195 appearances.
