Smith, ejected in the second quarter, proved to be a crucial missing piece.
Nine of New York's 11 3-pointers came in the first half. Duhon, David Lee and Larry Hughes attacked with drives to the basket after Smith, the NBA's leader in blocked shots, was given two technical fouls by referee Bob Delaney.
"[Smith] just can't put himself in that position again," Hawks coach Mike Woodson said.
Knicks coach Mike D'Antoni said Smith's exit "made things a little easier for us, in terms of matchups, because he can change the dynamics of the game."
Duhon said Smith's ejection helped the Knicks change their attack plan. With Atlanta missing its top shot-blocker, the Knicks began driving to the basket.
"It definitely makes a big difference," Duhon said. "Usually you come off the pick and roll to the rim, No. 5 is back there. If he doesn't stop you he's going to make you alter your shot.
"It was kind of a relief knowing when you went to the basket that he wasn't going to be there to knock it away."
Lee, often matched against former University of Florida teammate Al Horford, had 18 points and 17 rebounds.
Harrington, the former Atlanta star, had 23 of New York's first 46 points.
"Al got hot in the first half and carried us to keep us close until our offense got where it needed to be," Duhon said.
D'Antoni said Harrington's hot start gave the Knicks "a jump start."
"He gave us some confidence, and this is the best game he's had all season," D'Antoni said. "Everybody started pitching in, which helped us stay in the game and gave us a chance to beat these guys."
Harrington said Atlanta's 11-0 run made him think "Man, we better do something or we're getting blown out of the building."
The Knicks, who had lost six of seven, made 11 of 25 3-point attempts, gave the Hawks only their second loss in 10 home games. Harrington and Duhon each had four 3s.
The Hawks made only 4 of 23 3-pointers.
Smith apparently complained to referee Bob Delaney about Harrington's defense, drawing the technical with 6:21 remaining in the second quarter.
Smith walked to midcourt and continued his protests to Delaney, drawing a second technical foul and the ejection. Smith threw his mouthpiece to the floor on his way off the court and kicked a trash can in the hall leading to the locker room.
Smith was not available for comment after the game.
Johnson said Smith apologized to the team after the game.
"Those things happen," Johnson said of Smith's ejection. "When they do, other guys have to step up."
Williams said Smith's ejection was "absolutely" a surprise.
"You kind of hope a guy can contain himself in situations like that," Williams said. "I don't know what happened. It didn't look like anything too out of control. Obviously, the referee didn't like it."
New York led 48-42 when Smith was ejected.
Knicks guard Nate Robinson, who did not play Wednesday night in a loss at Orlando, was in uniform but again was held out.
Larry Hughes had 16 points for New York. Tony Douglas, with six points, was the only other Knicks backup to score.
Knicks F Danilo Gallinari was in uniform but did not play due to a sore right forearm. He started 17 of the team's first 19 games and said he hopes to play Sunday against New Jersey. ... Undefeated boxer Floyd Mayweather Jr. attended the game. ... Smith left with eight points and five rebounds.