Four months later, the Sixers have a shot at a playoff berth thanks to Miller and Smith.
Louis Williams also tied a career high with 18 points and Smith added 16 for the Sixers, who outscored Boston 55-36 in the second half to win their third straight game.
Philadelphia (33-45) improved to 28-27 since acquiring Miller and Smith and pulled to within three games of Orlando for the eighth and final Eastern Conference playoff spot.
The Sixers, who also trail the Indiana Pacers by two games, must win all four of their remaining games to have a chance. They host the Magic on Saturday.
"Me and Joe came over with the idea that we wanted to help the team, and I think we've shown great leadership," Miller said. "I think everybody is a little bit surprised."
Boston, meanwhile, is playing for lottery position. The Celtics have lost five in a row to fall to an Eastern Conference-worst 23-55.
"Obviously, we are not going anywhere," said Ryan Gomes, who scored a game-high 26 points to lead Boston. "We are trying to learn as a team, learning to play together. That's what we have to look forward to -- get better as individuals because the wins and losses aren't going to affect us at all."
After Sebastian Telfair's 3-pointer gave Boston a 65-50 lead early in the third, Philadelphia used a 16-4 run to cut the deficit to 69-66.
The Sixers tied it at 76 by the end of the third and took their first lead since the first quarter when Smith opened the final quarter with a jumper.
Gomes' 3-pointer pulled Boston to 92-89 with 4:31 remaining, but Miller scored four in a row to secure the win.
Gerald Green scored 22, Allan Ray had 15 and Rajon Rondo added 12 for the Celtics, who earlier in the day announced leading scorer Paul Pierce would miss the rest of the season with a sore left elbow.
Boston had only three players available on the bench, and Telfair left the game in the third quarter with a left ankle sprain.
"We're sponsoring tryouts tomorrow at 10:30," Celtics coach Doc Rivers joked.
Philadelphia also played without its leading scorer, Andre Iguodala, who sat out his second game with a strained lower back.
For the second day in a row, Philadelphia's backups picked up the slack.
Williams, who also scored 18 in a 90-86 win over Indiana on Tuesday, added a career-high eight assists, and Smith scored all his points in the second half to key the comeback.
Smith played sparingly in Denver, but the 12-year veteran who will be a free agent at season's end has had a resurgence with the Sixers. He scored six in a row in the fourth to erase an 80-78 Boston lead and give Philadelphia the lead for good.
"Being in Denver, I wasn't playing too much," Smith said. "Since I got here, I've been playing and I've been playing well. So now that I'm a free agent, I hope a lot of people recognize that."
- The 76ers won the season series 3-1.
- Philadelphia outscored Boston in the paint 60-28.
- Boston got just 14 points from its reserves.
- Boston C Kendrick Perkins had five points and 11 rebounds.
- Sixers C Samuel Dalembert had a career-high five assists.