Previously, Hasegawa played with the Anaheim Angels (1997–2001), and before that spent six years with the Orix BlueWave.

He converted 16 of 17 saves at the end of the season when Mariners closer Kazuhiro Sasaki was injured.

In his nine-season MLB career, Hasegawa compiled a 45–44 record with 33 saves and a 3.71 ERA in 517 games.

He holds the record for most appearances by an Asian pitcher in Major League Baseball ahead of Hideo Nomo.

In addition to selling real estate in Irvine, California, Hasegawa frequently appears on baseball-related television shows in Japan, and is a commentator for major league games shown in Japan on NHK.