Born in Newark, New Jersey in 1957, he attended Seton Hall University graduating in 1979.  Following his obtaining a law degree from Notre Dame University in 1982, he practiced as a trial lawyer in New Jersey until he moved to Maui in 1989.  From 1989 through 1997 Peter practiced with James Krueger focusing on personal injury litigation.

 In 1997, with his friend and colleague John O’Neill, they formed the firm of Cahill & O’Neill and remained partners until Chief Justice Mark Recktenwald swore in Cahill on September 17, 2012 to serve as a Circuit Court Judge.