He was part of the Millwall side that won the Football League Second Division championship with a club record 93 points in the 2000–01 season. He also reached two play-off semi-finals with Millwall in 2000 and 2002. Cahill was a central part of Millwall's run to the 2004 FA Cup Final, their first appearance in the showpiece in club history, scoring the winning goal in the semi-final against Sunderland, and securing a UEFA Cup place in the process. Cahill polled over 100,000 votes to win the FA Cup "Player of the Round" award for his performance during the semi-final victory. His last game for the club was against Manchester United in the final, which Millwall lost 3–0. Cahill made 249 overall appearances for "The Lions", scoring 56 goals in the process.