Thomas was acquired from the Suns in a trade last Feb. 19. Four days later in his only other game against Phoenix since the deal, he had 21 points and seven assists.
Smart had 10 points, 11 assists and 11 rebounds, and Kelly Olynyk added 21 points and nine rebounds.
Mirza Teletovic led Phoenix with 22 points, and Brandon Knight had 16. The Suns have lost 12 of 13 to drop to 13-28.
Boston improved to 21-19. The Celtics finished three points shy of their season-high points total, never letting the lead dip below 13 points in the second half.
Amir Johnson scored seven of Boston's first eight points and had nine in the opening period to help the Celtics take a 36-29 lead.
Sonny Weems' layup brought Phoenix to 46-42 with 6:26 to go in the second quarter, but a quick 6-0 Boston run pushed the lead back to double digits.
Olynyk scored 13 straight and all but three of Boston's last 16 points in the half that ended with the Celtics up 68-47.
Suns: Knight was 7 of 15 from the floor in his return to the lineup after missing Phoenix's loss Tuesday night in Indiana because of food poisoning. Knight is seven points away from 5,000 for his career and is one of nine NBA players averaging at least 19.0 points, 5.0 assists, and 1.3 steals. … Teletovic recorded his career-best seventh straight double-digit scoring game. … Alex Len (left hand sprain) missed his second straight game.
Celtics: Jared Sullinger took the microphone before the game to remind fans to celebrate Martin Luther King Jr. Day on Monday. "Without him, none of this would be possible," Sullinger said. … Thomas' emphatic block of Devon Booker's shot with 55.3 seconds left in the third quarter excited the crowd, but he was called a foul.
Phoenix is at Minnesota on Sunday. Boston is at Washington on Saturday night.