Passeau won a regular role with the Philadelphia Phillies, where he lost more games than he won, but he was a serviceable Pitcher and worthy of a Major League roster spot, and his advanced metrics were very good.
During the 1939 season, Passeau was traded to the Chicago Cubs, and his fortunes changed. Passeau was second that season in bWAR for Pitchers (6.0) and eighth in ERA (3.28), and in 1940, his first full year as a Cub, he was first in bWAR for Pitchers (6.9) and second in ERA (2.50). He won 20 Games that year, and he was named an All-Star five of the next six years, and except for the last year of that run (1946), he always had at least 14 Wins and had four years with a sub-3 ERA.
Passeau was a vital cog in the Cubs team that won the 1945 National League Pennant, and in the World Series, he pitched a one-hitter, but the Cubs lost the series to the Detroit Tigers.