Grier attended Davidson Day School in Davidson, North Carolina. As a junior, he threw for a national record 837 yards in a game, breaking the old record of 764. As a senior, he had 4,989 passing yards with a nation-leading and North Carolina record 77 touchdowns. He also rushed for 1,251 yards with 13 touchdowns. He was named the Parade All-American Player of the Year and Mr. Football USA. For his career, Grier had 14,565 passing yards, a state record 195 touchdown passes, 2,955 rushing yards and 31 rushing touchdowns.
Grier competed with Treon Harris to be Jeff Driskel's backup as a freshman in 2014, ultimately redshirting the season. As a redshirt freshman in 2015, he competed with Harris to be the starting quarterback. Although Harris started the first game of the season against New Mexico State, Grier received playing time, attempting 18 passes and completing 16 for 166 yards with two touchdowns. He rushed for 43 yards with a touchdown. Grier started the second game, against East Carolina, throwing for 151 yards, two touchdowns and an interception. On October 3, against the Ole Miss Rebels, he had the best game of his young career, going 24-of-29 for 271 yards and four passing touchdowns. He threw his four touchdowns in the first half; he was the first Gator quarterback since Chris Leak in 2005 to accomplish the feat. The game ended up being a 38–10 win for the Gators.
Grier received a one-year suspension, effective October 12, 2015, after it was revealed that he had tested positive for performance-enhancing drugs. He said the positive test stems from the over-the-counter supplement Ligandrol, and that he was not aware it was banned, while also admitting he never followed protocol to clear the supplement with team trainers. His appeal was rejected by the NCAA, and he would not be eligible to return until the sixth game of the regular season in 2016. On December 19, 2015, Florida announced that Grier planned to transfer to another school.
On April 6, 2016, Grier announced that he was transferring to West Virginia University. He sat out the 2016 season per NCAA transfer rules. Grier was the presumed starter for West Virginia heading into the 2017 season, but there were questions surrounding his eligibility, since half of his year-long suspension was served in conjunction with his transfer waiting period. On June 20, 2017, West Virginia head coach Dana Holgerson announced that the NCAA had granted Grier a waiver and he would be eligible to play in the season opener against Virginia Tech. In his first game with the Mountaineers, Grier threw for 371 yards, three touchdowns and an interception in a 31–24 loss to Virginia Tech. Grier threw for 352 yards and five touchdowns in a come-from-behind victory over then-No. 24 Texas Tech on October 14, and was named Big 12 Offensive Player of the Week. In the Mountaineer's 11th game of the season, against Texas, Grier left the game in the second quarter with a broken finger on his throwing hand. Grier had surgery on the broken finger and missed the remainder of the season, finishing with 3,490 passing yards and 36 total touchdowns on the year. At the conclusion of the season, Grier was awarded as Big 12 Offensive Newcomer of the Year.
On May 23, 2018, Grier announced he would be returning for his senior season rather than entering the 2018 NFL Draft. He was listed on preseason watch lists for several national awards, including the Maxwell Award,Walter Camp Award, and Davey O'Brien Award. Grier appeared in several preseason lists of potential Heisman Trophy candidates. At the 2018 Big 12 Media Days, he was announced as the conference's Preseason Offensive Player of the Year and a preseason first-team all-conference selection.
West Virginia finished their 2018 season with an 8–3 regular season record, and was invited to the Camping World Bowl; Grier announced that he would not play in the bowl game, in order to prepare for the 2019 NFL Draft. Grier would be named the winner of the 2018 Senior CLASS Award for FBS football as the outstanding senior student-athlete in that grouping of schools.
Will Grier is the oldest child of Chad Grier and Elizabeth Grier-Floyd. His father coaches football and was a former quarterback for the East Carolina Pirates. He has two brothers, Nash and Hayes, and a sister, Skylynn Floyd. All were popular users of Vine, the defunct video sharing service. Grier is married to Jeanne O'Neil Grier and their daughter, Eloise Marie Grier, was born in 2016.