Andre Iguodala hit a 3-pointer for the Nuggets with 26 seconds to play that brought them within 106-104, but Williams boosted the lead to four from the foul line with 17 seconds remaining.
Horford had 25 points and 12 rebounds with Smith adding 16 points and 13 boards. Atlanta bounced back from a loss to Cleveland on Friday that ended a six-game winning streak.
Lawson paced the Nuggets with 32 points while making 12-of-16 shots from the field. He was good on all five of his 3-point attempts.