Carey's seventh 3-pointer pushed the Lancers ahead just four minutes after TCU took its first lead of the game with 8:32 to play. After Carey's 3 from the right corner, Karl Ziegler and Jaylani Dublin each scored on putbacks and the Lancers made 9 of 10 free throws down the stretch.
But after Longwood turned the ball over on TCU's end of the court with 20.9 seconds left, TCU's Brandon Parrish hit a 3-pointer and Kyan Anderson hit two free throws to tie the game 79-79 with 7.1 seconds left.
Longwood won just its 16th win in a true road game since first playing a Division I schedule in 2004-2005.
It was Longwood's first-ever win over a Big 12 team in six tries.
Ziegler added 14 points and 10 rebounds for his first career double-double as Longwood, which plays in the Big South, moved to 1-1.
Anderson led TCU (0-2) with 17 points while Jarvis Ray scored 15 and Parrish 13.
The Lancers jumped out to 9-0 lead as the Horned Frogs missed their first five shots from the field and first four free throws while committing two turnovers. It wasn't until Ray's jumper with 14:50 to go that TCU got on the board. Longwood's largest lead of the game was 40-24 with 46 seconds to go before halftime, and the Lancers led 40-27 at the break behind Carey's 20 points.
The Frogs started the second half on a 13-0 run beginning with a Ray dunk and continuing with an emphasis on getting to the rim. Anderson had two layups in the run and Charles Hill, Jr., a putback, and the Frogs and Lancers were tied at 40 at the end of the run.
TCU was playing without freshman Hudson Price, who suffered a right foot injury in TCU's season-opening loss to SMU -- a game that Price started.