Thomas had the winning basket with 2.2 seconds left while scoring 13 of his game-high 28 points in the fourth quarter as the Celtics won 103-101 despite the Hawks overcoming a 16-point deficit in the fourth quarter.
The Hawks tied the score on a 3-pointer by Paul Millsap with 25.6 seconds remaining, but the Celtics (25-15) set up Thomas for the final shot and the diminutive guard penetrated and hit a 19-foot fallback jumper.
"Isaiah saved the day," said Horford, who left the Hawks to sign with the Celtics as a free agent last summer.
The Celtics, winners of six of seven games, made 17 3-pointers and got a boost off the bench from Kelly Olynyk, who had a season-high 26 points.
"We really wanted to dig this on out for Al," Olynyk said. "He deserved it and I'm glad we could get it for him."
Millsap and Tim Hardaway Jr. had 23 points to lead the Hawks, with Hardaway scoring 15 of his in the fourth quarter.
Atlanta got 31 of its 36 points in the fourth quarter from reserves.
"We were fresh and got it going," Hardaway said. "We dug ourselves a hole in the third quarter, but we battled back and showed heart. Unfortunately, we didn't get the win. IT (Thomas) did what he does at the end and that was it."
Horford had 10 points, six rebounds and six assists in his first game against his former team.
Horford received more boos than cheers during pregame introductions, but most Hawks fans showed their appreciate for the four-time all-star's nine seasons in Atlanta with a standing ovation after a video tribute between the first and second quarters.
"The (boos) weren't unexpected," Horford said. "It is what it is. I know a lot fans appreciated my time here and I appreciate them."
Thomas and Jae Crowder each had eight points in the third quarter as the Celtics, their lead down to four points at halftime from 15, moved out to a 20-point margin before the Hawks cut it back to 78-65 going into the fourth quarter.
The Celtics led 25-10 after making 7 of 10 early 3-pointers and it was 28-18 after the first quarter. They cooled off temporarily from behind the arc in the second quarter, though, and the Hawks was within 48-44 at halftime.
Olynyk, who was averaging 7.9 points, scored nine points in each of the opening quarters. Making 4 of 5 3-point attempts and hitting 7 of 8 shots overall.
The Celtics has 16 assists on 18 first-half baskets, 10 coming from behind the arc on 24 attempts.
Kent Bazemore sparked the Atlanta comeback in the second quarter, scoring 10 of his 12 points for the game. Millsap had 11 points at intermission.
Thomas finished with nine assists, six before halftime. Crowder had nine rebounds and Olynyk grabbed eight.
Dwight Howard had 11 points and eight rebounds for the Hawks. Delaney and Milsap each had six assists.
"We tied it up. I thought we were going to win it," Delaney said. "They have a great player in Isaiah Thomas. He takes the big shots for that team. He's a big fourth-quarter player."
NOTES: G Avery Bradley, the Celtics' second-leading scorer at 18 points per game, missed his fourth straight game because of a strained right Achilles tendon. ... Boston was also without C Tyler Zeller (sinus infection issues) and G/F James Young (ankle sprain). Coach Brad Stevens said Zeller, who has missed seven games, underwent tests and has felt better the past few days. ... F Amir Johnson returned to the Boston lineup after sitting out Wednesday against Washington because of a sprained ankle. ... G/F Mike Dunleavy Jr., acquired by Atlanta from Cleveland as part of the deal for G Kyle Korver, got in his first practice Thursday. ... The Hawks wore blue with lime green trim throwback uniforms from the early 1970s. ... Atlanta won the last three of four regular-season games against Boston last year, then took a six-game series in the opening round of the playoffs.