Leaf played college football with the Washington State Cougars, finishing as a finalist for the Heisman Trophy after his junior year.

He was selected as the second overall pick by the San Diego Chargers in the 1998 NFL draft after Peyton Manning, but his career was shortened due to poor play, bad behavior, injuries, and struggles with his work ethic and ability to stay focused.

[2] Leaf has worked as a Program Ambassador for Transcend Recovery Community, a group of sober living houses in Los Angeles, Houston, and New York.

Leaf did not know that Washington State had not reached the Rose Bowl since 1931, but later told Sports Illustrated that he immediately knew he wanted to accept a scholarship and play for Price.

Despite his strong early showing in the 1998 Rose Bowl, Washington State was defeated 21–16 by the eventual Associated Press national champion Michigan Wolverines.

[11] Peyton Manning and Leaf were widely considered to be the two best players available in the 1998 NFL draft,[8] and scouts and analysts debated who should be selected first.

[14][15][11] Tampa Bay Buccaneers coach Tony Dungy recalled that although his team did not need a quarterback, "Manning-Leaf was really split when you talked to people".

[16][19] Leaf's draft prospect profile described the player as "self-confident to the point where some people view him as being arrogant and almost obnoxious".

[21] Leaf gained about 20 pounds between the end of his junior season and the NFL Combine in February, which Jerry Angelo, one of six experts Sports Illustrated consulted on the choice, described as "a [negative] signal" about his self-discipline.

[23] San Diego's high hopes for Leaf were soon dashed as his rookie season was marred by poor behavior, starting with skipping the final day of a symposium mandatory for all NFL draftees and incurring a $10,000 fine.

[26] Three days before the Chargers' September 20 game against the Kansas City Chiefs,[27] Leaf was hospitalized for a viral infection that he attributed to an improperly cleaned artificial-turf burn.

[33] Leaf finished the season with 1,289 passing yards in ten games and a 45.3% completion percentage with only two touchdowns against fifteen interceptions, earning him an abysmal quarterback rating of 39.0.

[23] One month later, a fan heckled Leaf by singing lyrics from the Little River Band song "Lonesome Loser" and comparing him to failed NFL quarterback Heath Shuler.

[40] He was placed on injured reserve but made headlines in early November when he got into a shouting match with Beathard and a coach, resulting in a fine, a suspension without pay, and an apology from Leaf four weeks later.

[41][42] During his suspension, he was caught on video playing flag football at a San Diego park, a violation of his contract according to team management.

He suffered swelling in his left hand in addition to a chin gash that required stitches following a late hit from Raiders defensive tackle Regan Upshaw.

[48] In November, he publicly speculated that the Chargers would release him after the season,[49] and later that month, reports suggested that Leaf had lied about his wrist injury to get out of practice and play golf instead.

Leaf threw an interception on his fourth snap, led a touchdown drive in the Chargers' next series, and left the game with nearly a minute to go after straining a hamstring on a scramble.

[26] In the Chargers' final drive, with nearly two minutes remaining in the game, Leaf completed a 10-yard pass to Curtis Conway that was ruled six inches short of the end zone.

[52] On the final game of the season on December 24, Leaf made a 71-yard touchdown pass to Jeff Graham on the first play from scrimmage, but San Diego lost to the Pittsburgh Steelers 34–21.

[56] On March 2, 2001, two days after the Chargers released him, Leaf was claimed by the Tampa Bay Buccaneers, who were intrigued by his physical talent and planned to develop him more slowly, giving him time to watch and learn.

[59] Days later, Leaf got still another chance when the Seattle Seahawks signed him to a one-year contract, planning to let him develop slowly (as the Buccaneers had done) to allow his still-injured wrist time to heal.

While accepting responsibility for his poor behavior and play, Leaf said that his father telling Archie that the team did not help him was part of why Peyton's brother Eli Manning forced the Chargers to trade him to the New York Giants that year.

"[74] But in November 2008 he was put on indefinite leave, and resigned the next day from his coaching position at West Texas A&M for allegedly asking one of his players for a pill to help him deal with pain in his wrist from past injuries.

[75] Leaf's usage of painkillers dates back to 2002, when he took Vicodin from a boxing promoter after attending a match in Las Vegas.

"[84] In April, he wrote an article for The Players' Tribune titled "Letter to My Younger Self", describing his NFL career and life after its end.

[76] His younger brother Brady Leaf played quarterback for the Oregon Ducks football team behind Dennis Dixon from 2003 to 2006.

[94][95] However, his attorney Jeffrey A. Lustick successfully blocked the fugitive warrant extradition process, therefore legally allowing Leaf to go to Texas on his own.

[103] But on January 17, 2013, Leaf was remanded to Montana State Prison in Deer Lodge after being found guilty of "behavior that violated conditions of his drug treatment placement".

[105] On September 9, 2014, a Texas judge sentenced Leaf to five years' imprisonment, giving credit for time spent in prison in Montana.