Allen has been mentioned as one of the greatest CFL quarterbacks of all time after winning four Grey Cups with three different teams.

[5] Allen started playing football by the age of six for Valencia Park's Pop Warner in San Diego.

Allen joined the CFL as a free-agent in 1985, signed by the Edmonton Eskimos, and threw for 661 yards and three touchdowns in his rookie season as a back-up to Matt Dunigan.

In the 1987 season, Allen played in the Grey Cup championship game, replacing the injured starter Dunigan, and led the Eskimos to a Grey Cup victory, defeating the Toronto Argonauts 38–36, at Vancouver's BC Place Stadium.

Allen captured his first Grey Cup with the Lions (third of his career) throwing for 234 yards in a win over the Montreal Alouettes.

In Week 7, Allen became only the third quarterback in professional football history to pass for 60,000 yards against his former club, the BC Lions.

In Week 17, Allen dressed as the third quarterback (behind Michael Bishop and Romaro Miller) for a game against the BC Lions.

In Week 18, Allen dressed as the backup quarterback behind Michael Bishop and entered the game versus the Calgary Stampeders at the end of the second quarter.

On October 27, 2005, Damon reached the 5,000-yard passing plateau for the first time in his 21-year CFL career by posting a 34–11 victory against the Hamilton Tiger-Cats.

The win guaranteed Toronto a first-place finish in the CFL East plus home field advantage in the playoffs.

However, the Argonauts were defeated 33–17 in the 2005 East Final by the Montreal Alouettes, thereby thwarting their hopes of repeating their 2004 Grey Cup win.

In 2006, Allen started the opening home game of the season against the Hamilton Tiger-Cats and broke the middle finger on his right hand on Toronto's third play.

Allen was injured trying to break his fall after taking a late hit from Hamilton safety Wayne Shaw, who was penalized on the play.

Allen missed nearly a month and a half of action with a broken finger, returning on July 29 vs. the BC Lions.

On September 4, 2006, at the Labour Day Classic against the Hamilton Tiger-Cats, Allen broke Warren Moon's record (70,553 yards) to become professional football's all-time passing leader.

The record-setting completion, a 29-yard shovel pass to wide receiver Arland Bruce III, took place at 5:02 of the third quarter.

Allen's triumphant pose with the record-breaking football was captured by a photographer and later released by the Argos as a commemorative poster on their website.

On January 18, 2007, Allen underwent successful surgery to repair the middle finger of his (right) throwing hand and was back in the gym training, in less than 24 hours.

However, after Bishop went down with a wrist injury and McMahon failed to impress, Allen stepped into the starting role once again.