McConnell attended Chartiers Valley High School in the Pittsburgh-area borough of Bridgeville, Pennsylvania, where he played for his father, Tim.

[1][3] As a sophomore in 2011–12, he averaged 11.4 points, 4.4 rebounds, 5.5 assists, and 2.8 steals per game, helping the Dukes to a 16–15 record and earning third-team All-Atlantic 10 Conference honors and a spot on the A-10 All-Defensive Team.

[7] As a senior, McConnell was voted first-team All-Pac-12 and named to the Pac-12 All-Defensive Team, and he helped lead Arizona to another appearance in the Elite Eight.

[11] He played well during preseason, averaging 6.2 points and 4.8 assists in five games, earning himself a spot on the 76ers' opening night roster.

[18] On January 6, 2017, he had a career-high 17 assists in a 110–106 loss to the Boston Celtics, becoming just the fourth player in franchise history to hit that mark in a single contest.

His total was two off the Philadelphia's all-time record of 19 assists, initially set by Maurice Cheeks in 1987, and later matched by Dana Barros in 1995.

[19] McConnell hit a game-winning buzzer beater against the New York Knicks with a turnaround jump shot on the baseline on January 11, 2017.

[34] On May 2, 2024, McConnell scored a playoff career-high 20 points and 9 rebounds in a 120–98 series-clinching victory against the Milwaukee Bucks in Game 6 of the First Round.

[37] His father, Tim, is one of the most successful coaches in WPIAL (Western Pennsylvania Interscholastic Athletic League) history.

After many successful runs with the Chartiers Valley High School Boys and Girls basketball teams, McConnell was summoned by Bishop Canevin's athletic director, Dale Checketts, to become the head coach of the Bishop Canevin High School Boys basketball team, a position recently vacated by T.J's roommate at Duquesne University, Gino Palmosina.

His aunt, Suzie McConnell-Serio, was an All-American at Penn State (1985–1988), Olympic gold medalist and WNBA standout prior to being inducted to the Women's Basketball Hall of Fame in 2008.