The Nets had not defeated the Pacers since December 2008.
Williams produced 13 of his 30 points in the final period, during which New Jersey outscored the Pacers 31-17.
New Jersey won for the second time in eight games, also getting 17 points from MarShon Brooks and 14 from Gerald Green.
Indiana, which had won two straight, fell short in a bid for its 30th victory of the season. Paul George led Indiana with 22 points.