[2] Thayer helped the Joliet Catholic "Hilltoppers" to win consecutive Class 4A IHSA football championships in 1977 and 1978 as a junior and senior.

[7] Thayer was named an honorable All-American while at Notre Dame[1] and graduated with a degree in media communications in 1983.

[8][9] Thayer, however, was also one of the twenty-five players selected by the USFL's Chicago Blitz as part of the new league's 1983 Territorial Draft, which was held in January 1983,[10] and the Bears were unaware that Thayer had already agreed to a contract with the Blitz to play under head coach George Allen just hours before.

[9] According to Thayer, "They [the Bears] had told me before that I had been their fourth-round choice in a mock draft, but the Blitz was giving me first-round money, and it was guaranteed".

[5][12] He even made it as far as the 1984 USFL Championship Game as a member of the Wranglers,[13] but the team was defeated 23–3 by the Philadelphia Stars.

Thayer then signed with the Bears for the 1985 season,[11] joining a team, which also included fellow 1983 draft class members[9] Jimbo Covert, Willie Gault, Mike Richardson, Dave Duerson, Richard Dent, and Mark Bortz, that would go on to win Super Bowl XX.

He was part of a sports-talk program at WCBR-FM before signing a three-year contract with WMAQ-AM, replacing former teammate Dan Hampton, to work as color commentator alongside Wayne Larrivee and Hub Arkush for Bears' radio broadcasts.