Linebacker Myles Jack and defensive end Eddie Vanderdoes also rushed for touchdowns as the Bruins (9-3, 6-3 Pac-12) earned their first win at the Coliseum since 1997, retaining the Victory Bell by grinding out just their third victory over USC (9-4, 6-3) in 15 years.
Hundley and UCLA coach Jim Mora have never lost to the Trojans, and Jack had his latest big game in his first taste of the rivalry. After the freshman scored his seventh touchdown in four games, he also recovered Allen's fumble deep in UCLA territory early in the fourth quarter.
Anthony Barr then swatted the ball out of Kessler's hand from behind with 6 minutes left, and Cassius Marsh fell on the fumble near midfield. After Hundley rushed 23 yards up the middle on another nimble draw, Paul Perkins scored the clinching TD from 8 yards out with 3:50 to play.
The game didn't help the unlikely cause of Orgeron, who emerged as a legitimate candidate for the full-time USC job by revitalizing a stagnant USC program since replacing Lane Kiffin in late September.
Orgeron got the Trojans back into the national rankings, shocked No. 5 Stanford earlier this month, and made football fun again at USC -- but he also lost to Notre Dame and UCLA, the Trojans' two biggest rivals. Athletic director Pat Haden hasn't said when he'll pick a full-time coach.
Marqise Lee had six catches for 69 yards in what's expected to be the final home game for the USC junior, who won the Biletnikoff Award last year before struggling with injuries all season long.
The crosstown rivals had rarely been so evenly matched heading into their annual meeting. UCLA and USC hadn't been next to each other in the AP rankings for the game since 1976, and both schools are headed to bowl games after solid seasons.
Both teams wore their home jerseys at the Coliseum, sticking to tradition at the venerable arena they shared for decades. UCLA broke from that tradition two years ago and debuted an all-white uniform at the Coliseum, only to lose 50-0 in a game that got coach Rick Neuheisel fired.
Two hours before the game, USC's players and coaches gathered in a large circle at midfield in what's become a tradition under Orgeron. The Trojans exchanged angry words with several Bruins attempting to warm up on the field, but security and coaches preventing anything but insults from being thrown.
The Coliseum crowd roared for USC's seniors before the game -- and then went crazy when former linebacker Willie McGinest led the rest of the Trojans onto the field after ripping off his sweatshirt to reveal his old No. 55 jersey and pulling on a helmet.
UCLA made a 48-yard scoring drive late in the first quarter, with Hundley calling plays at the line of scrimmage and scrambling repeatedly to set up Jack's 3-yard TD run. Jack scored 6 touchdowns on 35 carries in the Bruins' previous three games when the freshman was pressed into offensive service by injuries to several UCLA tailbacks.
UCLA added an 80-yard scoring drive early in the second quarter ending in a TD run by the 305-pound Vanderdoes, who committed to USC and Notre Dame before switching from the Irish to UCLA earlier this year.
UCLA's offense was stopped on fourth down at the USC 12 late in the first half, but the Bruins went up 21-7 early in the second half on an 11-yard TD draw by Hundley, who went in untouched.
The Trojans responded with a 22-yard TD pass to Grimble, but the Bruins got another huge kick return from Ishmael Adams and scored less than three minutes later. Hundley took it in on a 5-yard run, beating USC's Devon Kennard to the corner for his second TD.