Eason transferred to the University of Illinois and sat out the 1980 season. He was 6 feet, 4 inches tall, and weighed 205 pounds when he took over as the starting quarterback for the Fighting Illini in 1981. His first start for Illinois matched Eason against Pitt's quarterback Dan Marino; the Illini lost, 26–6, but Eason made an impressive debut as he completed 23 of 37 passes for 207 yards (slightly better than the 204 yards passing by Marino in the game). In his first season with the Illini, Eason led the Illini to a 7-4 record, but the team was ineligible to play in a bowl game due to sanctions imposed by the Big Ten Conference. Eason completed 248 of 406 passes (61.1%) for 3,360 yards and 20 touchdowns. The Illini ranked third in the nation in passing in 1981, as Eason led the Big Ten in passing efficiency and total offense and set nine conference records, including records for total offense, completions, passing yardage, and passing touchdowns. He also edged out Art Schlichter as the quarterback on the Associated Press' All Big-Ten football team.