[2] Rated as one of the top quarterback recruits in his class, Costello committed to Stanford University to play college football.

[7][8] He remained the starter throughout the rest of the season and finished with 1,573 passing yards, 14 touchdowns and four interceptions and took the Cardinal to the Pac-12 Championship game.

He holds the SEC Single-Game passing record with 623 yards against the defending national champions, LSU Tigers.

[12] On August 1, 2021, Costello signed with the Los Angeles Chargers after going undrafted in the 2021 NFL draft.

[19] The Stars folded when the XFL and USFL merged to create the United Football League (UFL).