The Aces reversed three straight losses to the Clippers, including a 59-33 drubbing in last year's state final and a 60-46 defeat in the District 1 final earlier this month at Villanova's Pavilion.
"I can't even explain this kind of feeling," senior wing guard Raheem Hall said. "Getting this gold medal is such a great accomplishment. We worked real hard for this to happen."
Downer's last title, in 2006, also came at the Giant Center. In 1996, powered by Kobe Bryant, statewide glory came at Hersheypark Arena.
B.J. Johnson, a 6-foot-7 swingman and Syracuse recruit, paced Lower Merion with 22 points and 11 rebounds. He netted 12 points in the first half, including two three-pointers in the opening seven minutes.
Said Downer: "He worked so hard to gain this redemption. It couldn't have happened to a better player, one of the best we've ever had here. I'm so happy for him."
Hall contributed eight of his 12 points in the third quarter. Junior lead guard JaQuan Johnson chipped in six points, three assists, and two steals.
Yohanny Dalembert, a 6-foot-8 senior center, posted 10 points, seven boards, three assists, three steals, and two blocked shots.
Rondae Jefferson, a 6-7 swingman and Arizona recruit, paced Chester (28-4) with 18 rebounds, 12 points, five assists, two blocks, and two steals.
In winning their seventh state title, the Aces denied the Clippers a ninth crown and snapped their streak of 78 consecutive wins against in-state opponents.
Down by 20-18, the Aces (30-3) closed the first half with a 10-0 burst to surge ahead, 28-20.
Key for Lower Merion was slowing the faster Clippers with a four-corners offense.
"We thought we played too fast in the district final," Downer said. "We wanted to spread them out this time, create as much space as possible."
Late in the third quarter, sub Baird Howland's right-wing three gave Lower Merion a 43-35 advantage.
Early in the fourth, Dalembert's follow dunk and Hall's fastbreak layup produced a 51-40 cushion. Later, Dalembert's left-handed slam bumped the lead to 55-44.
Chester's Darius Robinson, a senior guard and three-point threat, played little after suffering a foot injury in the second quarter.
Chester 14 6 16 11 – 47
Lower Merion 12 16 15 20 – 63
C: Conrad Chambers 2, Darius Robinson 6, Rashan DeJarnette 4, Richard Granberry 10, Rondae Jefferson 12, James Kirksey 8, Isaiah Warren 3, Khalil Williams 2.
LM: JaQuan Johnson 6, Raheem Hall 12, Yohanny Dalembert 10, B.J. Johnson 22, Justin McFadden 5, Baird Howland 8.
Contact Rick O'Brien at firstname.lastname@example.org. Follow @ozoneinq on Twitter. Read his blog, "The O'Zone," at www.philly.com/ozone