Nicknamed "J-Mac," McElwain was diagnosed with autism at a young age, and he did not talk until he was five years old. As a child, he had trouble interacting with other kids, but during his teenage years he began to develop social skills and make friends. McElwain had a passion for basketball, so Greece Athena High School basketball coach Jim Johnson appointed him manager of the team.

On February 16, 2006, Greece Athena was playing Spencerport High School for a division title. Greece Athena got a large lead, so Johnson decided to let McElwain play in the last four minutes. After initially missing two shots, McElwain made six three-point shots and one two-pointer. After the final buzzer rang, the crowd dashed onto the court in celebration of McElwain's amazing game.

McElwain won an ESPY Award for the Best Moment in Sports in 2006. To win that award, McElwain beat out Kobe Bryant's 81-point-game and the George Mason Patriots' run to the Final Four. The speech that Jason gave upon winning the award was written for him by his older brother. The theme of the speech was about dreams coming true.

Following his rise to fame, Jason McElwain wrote a book titled The Game of My Life. The book is written mainly by Jason, but includes sections written by his family, coach, and teammates. Editorial reviews were left by celebrities such as Magic Johnson, Doug Flutie, and Tony Dungy.