Garrett Dellinger attended Clarkston High School in Michigan, where he was a four-year starter on the football team. He started at right guard as a freshman and then shifted to right tackle as a sophomore, before moving to left tackle as a junior and senior. Dellinger was rated as a four-star offensive tackle by ESPN and the 31st best player nationally. He was twice named to the All-Oakland Activities Association Team and also was named an Under Armour All-American. During his senior season he elected to commit to LSU to play football for the Tigers.
As a true freshman in 2021, Dellinger played in 12 games with four starts, three at left tackle and one at left guard. He showed off his versatility as a sophomore when he played in nine games, starting six times (one at center and five at left guard). He missed five games during the season with an injury, but still played a total of 397 snaps on the year. In 2023, Dellinger settled in as the starting left guard and started all 13 games there. The Tigers offense topped 40-points and 500-yards of total offense nine times. He finished the season as a finalist for the Joe Moore Offensive Line of the Year Award.
In 2024, Dellinger started seven games at left guard before suffering an ankle injury.