Junior guard Jake Silpe scored 14 and played a strong floor game, and sophomore center Tim Perry added 13 points and 12 rebounds for Cherry Hill East (16-7, 3-3).
"They made three key plays down the stretch to get the lead, and then they made their foul shots to keep it," Cherry Hill East coach Dave Allen said. "Congratulations to them on winning the division. They deserve it."
East led for most of the game behind Silpe, who collected six rebounds, three steals, and three assists.
The Cougars held a 47-45 lead with possession with 1 minute, 13 seconds to play.
But a missed layup by the visitors kicked off a stunning sequence in which Eastern scored the final nine points and earned its second division title in three years.
The Vikings also reached the 20-win mark for the fourth straight season.
"It's always a tough game because no one's sets work," Barr said. "We know each other so well. We knew we had to pick up it up on defense when it got late in the game."
Barr scored on a layup with 55 seconds left to tie the score, then immediately stole the inbounds pass and added another layup to give Eastern a 49-47 lead - the Vikings' first edge since 5-4.
Cherry Hill East had a chance to tie but missed two free throws with 41 seconds remaining.
Barr went 5 for 6 from the foul line in the final 19 seconds to seal the victory for the Vikings.
"We know them, and they know us," said Eastern coach Joe Murphy, whose team could play Cherry Hill East for the third time in the third round of the South Jersey Group 4 tournament. "Sometimes I think they know our plays better than our guys do.
"So it comes down to making plays, and Alex made plays for us down the stretch."
Cherry Hill East 12 9 15 11 - 47
Eastern 11 9 11 23 - 54
CHE: Jake Silpe 14, Austin Wetzler 7, Tim Perry 13, Tom Smierciak 4, Phil Cunha 9.
E: Matt Klinewski 16, Alex Barr 26, Logan Edwards 8, Tom Flacco 2, John Scalindro 2.
Eastern's Alex Barr looks back at the game.