Williamson completed 16 of 27 passes and connected with Rashawn Proctor on a 4-yard TD pass in the third quarter and found Mixson with 7:04 remaining for a 28-20 lead.
But South Carolina State recovered a fumble on a punt return with 3:36 remaining and went on a six-play, 29-yard drive to get within 28-26 on Dondre Brown's 1-yard TD run. But the Bulldogs' 2-point conversion was no good as quarterback Adrian Kollock Jr. was tackled at the 2.
Williamson hit Jack Willenbrock on a 12-yard TD pass in the first quarter, but the Pirates (4-4, 4-2 Mid-Eastern Athletic) trailed 20-14 at halftime.
Caleb York threw a 23-yard TD pass to Kanome Mcintosh and a 13-yarder to Bishop York, but S.C. State (3-4, 3-1) was hurt by a missed point-after attempt in the second quarter and a missed a 26-yard field goal try in the third.
Ranking the rivalry games by playoff impact
The stakes are always high in season-ending rivalry games, but this season rivals are standing in the way of three Power 5 teams finishing undefeated.
Why the stakes have never been higher for Urban Meyer and Jim Harbaugh
For the Wolverines, a win means a likely CFP berth and long-awaited revenge. For the Buckeyes, it will change the story of a chaotic season.
Jerome Souers stepping down as Northern Arizona's coach
Northern Arizona coach Jerome Souers is stepping down as head football coach after 21 years of leading the Lumberjacks.
Higdon implies Michigan will beat Ohio State
Michigan RB Karan Higdon says he agrees with his coach that a win vs. Ohio State is guaranteed this weekend.
Tigers' offense has to rise to occasion vs. Aggies
Thinking Out Loud's Marcus Spears and Greg McElroy discuss how important the LSU-Texas A&M matchup is for both teams.
Will Georgia Tech's offense give UGA trouble?
Thinking Out Loud's Marcus Spears talks about the challenge the Yellow Jackets' triple option can present to the Bulldogs.