Williams came off the bench and played 29 minutes, while contributing 15 points, three rebounds and three assists.
He had missed the previous 11 games due to a rib cartilage fracture. The Nets struggled without him, ranking 25th in offensive efficiency (97.3 points per 100 possessions) in January.
"I was happy to be back out there, but I was happier to get a win," Williams said. "It's tough when your team is struggling and you can't be out there with them. I didn't feel winded at all."
Because Williams had played just four minutes over the past 13 games due to injury and third-stringer-turned-backup Darius Morris has been mostly ineffective, Jack has averaged 37.4 minutes over that span.
Still, Jack played 40 minutes against the Clippers and scored the game-winner with 1.3 seconds remaining.
The Nets (19-28) snapped a four-game skid with Monday's win, but are now just 3-12 in their past 15 games.
Williams, 30, entered Monday averaging 13.9 points and 6.3 assists per game on 39.6 percent shooting this season. He had been coming off the bench before getting injured.