Green had 120 yards on 13 carries and Royster added 105 on 17.
But it was Clark who led a pair of third-quarter drives that decided the game. He scored 1-yard touchdowns to end each and stretch what had been a 7-3 halftime lead to 21-3.
Penn State (4-1, 1-1 Big Ten) relied on its ground game to bounce back from last week's upset against Iowa, battering Illinois with 338 yards rushing.
Illinois (1-3, 0-2) was led by quarterback Juice Williams' 20-for-36 passing for 263 yards. He also ran for a score.
Illini receiver Arrelious Benn had five catches for 96 yards.
(This version CORRECTS APNewsNow. No. 15 Penn State 35, Illinois 17. corrects Williams' passing totals in 5th graf.)