The young red-headed, freckled Whitnall, known as "Banger", "Big Red" or simply "Red", quickly established himself in Carlton's best 22, kicking four goals on debut, and missing only a handful of games in 1997 and playing every game in 1998 and 1999. He became a vital part of Carlton's forward line, and in Round 17, 1998 against the Western Bulldogs, the then-19-year-old Whitnall became one of the youngest players to kick 8 goals in a game. It was in this match that he well and truly had established his presence. Whitnall finished with 46 goals for the 1998 season, followed by 55 goals in 1999.
In 2005, Whitnall turned around his previous weight problems and came out looking as fit as he had ever been. He performed very well during 2005 and 2006, playing predominately as the senior member of the Blues' young backline. He capped off a solid 2006 with victory in the Blues' best and fairest (the John Nicholls Medal), and at the age of 27, Whitnall became the second youngest Carlton player (after Robert Walls) to play 200 AFL games.
A long term member of the leadership team, and widely regarded as having one of the best football brains in the game, Whitnall first took over the captaincy of the team during Anthony Koutoufides' hand injury in 2006, and became the full-time captain in 2007 after Koutoufides relinquished the position. However he struggled in 2007 with ongoing problems with a degenerative condition in his knee. At the completion of the 2007 season, concerns by the club about his degenerative knee being able to stand up to the pressures of AFL football forced Whitnall into retirement, bringing his 11 season, 216 game career at Carlton to an end at 28 years of age.
Whitnall was recruited by NTFL side the Palmerston Magpies after being delisted by Carlton and played in two of the club's matches in the 2007/08 season, kicking several goals.
In the 2008 winter season, Whitnall returned to play for original club Lalor in the NFL. He played there in 2008 and 2009. In 2010, Whitnall shifted to the EDFL's Craigieburn Football Club where he served as playing assistant coach for two seasons; in 2011, Whitnall was the EDFL's A-Grade leading goalkicker, after kicking fourteen goals in the final round. Whitnall has played and coached at Glenroy, also in the EDFL, since 2012.
Whitnall also plays cricket at a suburban level for the Lalor Warriors Cricket Club in the North Metro Cricket Association, where he is an opening or top order batsman.