Hill, a fifth-year senior, has sustained three season-ending injuries, appearing in only one game last season, five in 2014 and six in 2012.
In 2013, he appeared in 13 games and completed 53.9 percent of his passes for 2,938 yards with 19 touchdowns and 14 interceptions. Hill also rushed for 1,344 yards that season. He has 31 career passing touchdowns.
Mangum, a sophomore, gained national attention last season after back-to-back game-winning touchdowns in the final minute. When Hill went down early in the fourth quarter of the season opener against Nebraska, Mangum tossed a game-winning Hail Mary. A week later he threw another game winner in the closing minute against Boise State.
Hill was diagnosed with a Lisfranc fracture and missed the remainder of the season. Mangum went on to throw for 3,377 yards and 23 touchdowns to 10 interceptions en route to a 9-4 season.
"We talked as a staff this weekend, and Taysom's going to be our starter," offensive coordinator Ty Detmer told the media Tuesday. "He's 100 percent healthy, and he's a fifth-year senior. He turned 26 today. He's got the experience and the leadership."
Detmer said Mangum has "handled it great" and made it a tough decision.
BYU opens the season Sept. 3 against the Arizona Wildcats in Glendale, Arizona.