Mitch McGary was born on June 6, 1992, and grew up in the Chesterton, Indiana, area. He played in YMCA and Amateur Athletic Union (AAU) basketball leagues as a youth, and his father, Tim, who had never played organized sports, coached him until fourth grade. McGary's mother is named Valerie. When McGary was younger, his father thought his athletic future was in baseball. McGary played on the 2007–08 Chesterton High School junior varsity team as a freshman, while eventual three-time Michigan Wolverines captain Zack Novak was a senior on the varsity team. That year he also was a 6-foot-6-inch (1.98 m), 190-pound (86 kg) freshman tight end on the high school football team, but his father made him quit football as he continued to grow. His local Indiana SPY Players AAU basketball team included future Michigan teammates Max Bielfeldt and Glenn Robinson III, son of Glenn Robinson. McGary joined the varsity basketball team the following season and played two years. McGary is afflicted with attention deficit hyperactivity disorder. He eventually transferred to Brewster Academy, a prep school in Wolfeboro, New Hampshire, to repeat his junior year because his parents thought the discipline would be good for him. Before transferring to Brewster, McGary was the tallest kid in his school and in the basketball conference that he played in. McGary shoots left-handed.
In the third game of the season and the third of his career, McGary posted a game-high 9 rebounds in 17 minutes of play off the bench against Cleveland State on November 13. McGary also went 3-for-3 on his field goals. McGary reached double figures in scoring for the first time December 4 against Western Michigan (Michigan's eighth game of the season) when he scored 10 points on 5-for-5 shooting in 14 minutes of play off the bench. He had his first 10-rebound game on December 11 against the Binghamton Bearcats. On December 20, he posted his first double-double with a then career-high 11 rebounds and 10 points as well as a then career-high 3 steals in just 18 minute of play against Eastern Michigan. On December 24, he was recognized as Big Ten Freshman of the Week. On January 6, McGary tied then career high with 11 rebounds, tied a then career high 2 assists and set a career high with 3 blocked shots against Iowa. On January 28, Michigan was ranked number one in the AP Poll with 51 of the 65 first place votes. It marked the first time Michigan ranked atop the AP Poll since the Fab Five 1992–93 team did so on December 5, 1992.