Facing the team he won two championships with, James helped his Cavaliers to a 122-119 overtime win as part of the NBA Global Games in Rio de Janeiro.
James could only score seven points and provide eight assists, but he said it was special to play against his former team.
"For me it was a special moment to be back out there, competing against my old team-mates," he said.
"I didn't get that awkward feeling but I did get a lot of memories came back, the things that we accomplished with the team-mates and the coaching staff, that's there.
"It was definitely great to see them."
The Cavs' other star recruit, Kevin Love, scored a game-high 25 points and pulled down seven rebounds.
For the Heat, Chris Bosh scored 19 as their record this pre-season slipped to 0-3.
It appeared James briefly forgot he was no longer at the Heat, setting a pick on team-mate Matthew Dellavedova.
Champions the San Antonio Spurs overcame Fenerbahce Ulker 96-90 thanks to Tim Duncan (23 points) and Tony Parker (22).
The New York Knicks were 96-80 winners over the Boston Celtics and the Memphis Grizzlies beat the Atlanta Hawks 93-88.
Pau Gasol's double-double of 20 points and 11 rebounds guided the Chicago Bulls to a 91-85 win over the Milwaukee Bucks.