Neither player held more than a 1-up lead during the match at Point O'Woods Golf & Country Club. Rain on Sunday forced the final to be pushed back a day.
Hack, of Sanford, Fla., joined 2007 NCAA champion Jamie Lovemark as the only players to win the Western Junior and Western Amateur in back-to-back years since the Western Golf Association started the national junior championship in 1914.
Lovemark, who withdrew from this year's Western Amateur after the first round due to injury, won the Western Junior in 2004. He became the youngest winner of the Western Amateur a year later, five months shy of his 18th birthday.
Hack will turn 18 on Sunday.
He took the lead for good on the par 4, 420-yard 16th hole when he dropped a wedge shot 8 feet from the pin and sank his birdie putt. That gave him a 1-up advantage over Prugh.