In 2002, with arguably one of the greatest junior teams ever assembled, Greenhalgh and future professionals Craig Conn, Billy Dee Smith, Matt Vinc, Athan Iannucci, Rory Glaves and Mike Hominuck, led the Athletics to an undefeated season and their second league championship in a row.
In Greenhalgh's final season, the Athletics won their third straight league championship and defeated the Burnaby Lakers to win their second Minto Cup in three years.
Greenhalgh was a five-year starter at Holy Cross Catholic Secondary School, where along with fellow National Lacrosse League players Craig Conn, Matt Vinc, and Billy Dee Smith, won 2 OFSAA championships, and created a dynasty at the school for many more years in the sport.
After high school, Greenhalgh played for Cornell University, where he was named 2005 Ivy League Player of the Year.
Greenhalgh was selected by the Los Angeles Riptide in the second round (19th overall) in the 2008 MLL Supplemental Draft.