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.
Joe Johnson led Atlanta with 29 points. Marvin Williams added 20 points and Horford added 17 points and 14 rebounds.
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.
NOTES: 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.
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