Sanchez entered spring practice after the 2008 season as the front-runner to take over the starting quarterback position, but faced strong competition from redshirt freshman Aaron Corp and Mitch Mustain, a transfer from Arkansas, where he had been the starting quarterback; Mustain, like Sanchez, was named the top quarterback in the nation upon the conclusion of high school career in 2006.
By the end of spring practice, Carroll announced Sanchez would be the starting quarterback heading into the fall.
During the first week of fall camp, Sanchez dislocated his left kneecap during warm-ups prior to practice; trainers immediately put the kneecap back into place.
After missing nearly three weeks, Sanchez was cleared to play in the opener against Virginia.
Before the opener, Sanchez was contacted by USC's previous three quarterbacks—Carson Palmer, Leinart and Booty—who wished him well and offered general advice.

In the opener at Virginia, Sanchez threw for a career-best 338 yards, completing 26 of his 35 passes for three touchdowns and one interception.
The Davey O'Brien Foundation named him the O'Brien Quarterback of the Week and his performance garnered early Heisman discussion.
The Trojans suffered a stunning 27–21 loss against Oregon State on September 25.
By season's end, the Trojans' lone loss was enough to remove them from contending for the BCS National Title and instead they were to play in the Rose Bowl against Penn State.
The Trojans defeated the Nittany Lions 38–24.
Sanchez won the 2009 Rose Bowl Most Valuable Player award for his performance on offense; his 413 passing yards ranked second in the history of the Rose Bowl and fourth in Trojan history.
With Sanchez starting all thirteen games, the Trojans ended the season 12–1 and ranked number two in the Coaches' Poll and number three in the AP Poll.
Sanchez finished the season with 3,207 yards passing, 34 touchdowns, second most in Trojan history, behind Leinart, and 10 interceptions.

Upon the conclusion of the Rose Bowl, Sanchez stated it would be "hard to say goodbye to [USC].
I don’t think I can do it."
However, with the subsequent announcement that other NFL-caliber quarterbacks, such as Sam Bradford, Tim Tebow, and Colt McCoy had decided to stay in school, rumors arose that Sanchez would use the opportunity to enter the 2009 NFL Draft.
On January 15, Sanchez announced his plans to forgo his final year of college eligibility and enter the 2009 NFL Draft, although he continued as a USC student and completed work on his degree in the spring of 2009 while preparing for the draft.

Sanchez became the first USC quarterback to leave school early for the NFL since Todd Marinovich did so after the 1990 season.
During the press conference, head coach Pete Carroll made it clear that he did not agree with Sanchez's decision, and advised him of the low success-rate of quarterbacks who left college early.
Despite the public disagreement, the two remained close afterward.
Sanchez entered spring practice after the 2008 season as the front-runner to take over the starting quarterback position, but faced strong competition from redshirt freshman Aaron Corp and Mitch Mustain, a transfer from Arkansas, where he had been the starting quarterback; Mustain, like Sanchez, was named the top quarterback in the nation upon the conclusion of high school career in 2006. By the end of spring practice, Carroll announced Sanchez would be the starting quarterback heading into the fall. During the first week of fall camp, Sanchez dislocated his left kneecap during warm-ups prior to practice; trainers immediately put the kneecap back into place. After missing nearly three weeks, Sanchez was cleared to play in the opener against Virginia. Before the opener, Sanchez was contacted by USC's previous three quarterbacks—Carson Palmer, Leinart and Booty—who wished him well and offered general advice.
In the opener at Virginia, Sanchez threw for a career-best 338 yards, completing 26 of his 35 passes for three touchdowns and one interception. The Davey O'Brien Foundation named him the O'Brien Quarterback of the Week and his performance garnered early Heisman discussion. The Trojans suffered a stunning 27–21 loss against Oregon State on September 25. By season's end, the Trojans' lone loss was enough to remove them from contending for the BCS National Title and instead they were to play in the Rose Bowl against Penn State. The Trojans defeated the Nittany Lions 38–24. Sanchez won the 2009 Rose Bowl Most Valuable Player award for his performance on offense; his 413 passing yards ranked second in the history of the Rose Bowl and fourth in Trojan history. With Sanchez starting all thirteen games, the Trojans ended the season 12–1 and ranked number two in the Coaches' Poll and number three in the AP Poll. Sanchez finished the season with 3,207 yards passing, 34 touchdowns, second most in Trojan history, behind Leinart, and 10 interceptions.
Upon the conclusion of the Rose Bowl, Sanchez stated it would be "hard to say goodbye to [USC]. I don’t think I can do it." However, with the subsequent announcement that other NFL-caliber quarterbacks, such as Sam Bradford, Tim Tebow, and Colt McCoy had decided to stay in school, rumors arose that Sanchez would use the opportunity to enter the 2009 NFL Draft. On January 15, Sanchez announced his plans to forgo his final year of college eligibility and enter the 2009 NFL Draft, although he continued as a USC student and completed work on his degree in the spring of 2009 while preparing for the draft.
Sanchez became the first USC quarterback to leave school early for the NFL since Todd Marinovich did so after the 1990 season. During the press conference, head coach Pete Carroll made it clear that he did not agree with Sanchez's decision, and advised him of the low success-rate of quarterbacks who left college early. Despite the public disagreement, the two remained close afterward.
Mark Sanchez