The Philadelphia Phillies traded for him in January 1910 due to finding this out, acquiring Brennan and Bob Ewing for Harry Coveleski and Frank Corridon.

[2] Brennan made his major league debut on May 19, 1910, and played in 19 games his rookie season, finishing the year with a 2–0 record and a 2.33 earned run average (ERA).

In 1912, he rejoined the Phillies' major league roster full-time, and pitched for them until a late-July bout of Diphtheria sidelined him for the rest of the season.

In one game against the New York Giants, manager John McGraw threw insults at Brennan while he was pitching.

After retiring, he served as head coach of the Fulton High School baseball team for two decades, and lived in Kansas City, Missouri until his death in 1962.