Wearing guernsey number 11 and nicknamed the "Flying Doormat" due to the matted appearance of the constantly disarranged long portions of his extreme "comb over" hairstyle. He was recruited from Jacana at the age of 19 as a half-back flanker. Doull was a safe mark, a dependable kick and a footballer who rarely made a mistake.
Doull, shy and extremely reserved, did not give interviews and always preferred to stay in the background. He won Carlton's Best & Fairest in 1974, 1977, 1980 and 1984 and played in four Carlton premiership sides – 1972, 1979, 1981 and 1982, winning the Norm Smith Medal in 1981, and also played in the losing Grand Finals of 1973 and 1986. Doull was also a regular State of Origin representative. In 2009 The Australian nominated Doull as one of the 25 greatest footballers never to win a Brownlow Medal.
He is often remembered as being harassed by Carlton scarf-wearing streaker Helen D'Amico in the 1982 Grand Final between Carlton and Richmond. This incident was the focus of a recent installment of the Toyota Memorable Moments advertisement (with D'Amico appearing at the end), and is captured in Jamie Cooper's painting the Game That Made Australia, commissioned by the AFL in 2008 to celebrate the 150th anniversary of the sport,  Both Doull and D'Amico posed for a photo together 25 years later, making it clear they had long since settled their disagreements.
Doull's trademark was his greying beard and the navy blue and white headband with which he kept his thinning long hair in place. Never reported by the umpires for foul play, he was noted for his determination to play the ball rather than the man, rare in an era of occasionally brutal clashes. Brent Crosswell wrote: "Doull's game has a moral purity about it, and that is why opponents have always found it extremely difficult to be unfair to him. It would have shamed them."
Only once did he appear to lose his temper. In a match against Essendon played late in his career, in 1983 a frustrated Cameron Clayton snatched off his ancient, faded headband and threw it into the crowd at Waverly Park. Incensed, the lumbering Doull went berserk and threw his opponent to the ground and had to be dragged away from the clash by his team mates. This incident was also recreated in his Toyota Memorable Moments advertisement, except that he remained his usual docile self when he was supposed to lose his temper.
By the end of his career, he had played 356 games, then a club record, and since surpassed only by Craig Bradley. Doull holds the current club record for most consecutive games played, with 162 matches played between 1971–1978; he actually missed two club games due to representing Victoria in interstate matches during this streak, but the AFL has formally included such games within a player's consecutive games streak since an amendment to its interpretation in December 2012. Doull kicked just 22 goals over his 18-year career.