Avellini played college football at the University of Maryland and was a sixth round selection of the Bears in the 1975 NFL draft.
The Bears showed improvement as they tied the Minnesota Vikings for the NFC Central title at 9–5 and earned a trip to the playoffs as the wildcard team, where they were handily beaten by the Dallas Cowboys.
This caused coach Neill Armstrong to make a switch at quarterback, inserting veteran Mike Phipps into the starting role.
[2] Phipps, for whom the Bears had traded their first-round pick in the 1978 draft to acquire from the Cleveland Browns, won 3 out of the final 4 games, to establish himself as the starter for the next season.
In 1984, with the Bears starting 2–0, Jim McMahon was injured and Mike Ditka inserted Avellini as the starter for a road game against the Green Bay Packers.
Avellini made a brief comeback in 1986 with the Dallas Cowboys, starting three preseason games, but was released at the final cut deadline.
In October 2013, a DuPage County grand jury indicted Avellini on felony drunken driving charges a week after his sixth DUI-related arrest since 2002.