His affiliation with Glenelg began very young; he developed a lifelong friendship with Chris McDermott when they played together in the under-10s team.
He began his senior career with Glenelg in the South Australian National Football League (SANFL) at age 17 in 1981, and played 116 games, kicking 266 goals (also playing 20 games and kicking 24 goals in night series/pre-season competition), and also won three straight best and fairest awards.
Under the coaching of 1961 Magarey Medallist John Halbert, Kernahan was a member of Glenelg's losing Grand Final teams to Port Adelaide in 1981 and Norwood in 1982.
Due to the rules of the SANFL, any player who receives a suspension during a season is ineligible to win the Medal and the award was won by North Adelaide's Tony Antrobus who polled 35 votes.
Kernahan was also twice leading goal kicker for The Bays, in 1983 and 1984 and in 1985 he was awarded the Jack Oatey Medal as best on the ground in the club's premiership win over North Adelaide.
Kernahan was signed by Carlton in the Victorian Football League (VFL) in 1981, but he didn't move to the club until 1986 due to his ambition to play in a premiership with his best mates at Glenelg.
He kicked 10 goals in a losing side against Victoria at Football Park in Adelaide in 1984, winning the Fos Williams Medal as South Australia's player of the match.
He became widely regarded for his leadership as captain, with former club chief executive Ian Collins describing him as "on and off the field, the greatest leader [Carlton] has ever had".
After taking a strong pack mark, Kernahan heard the siren go, steadied himself, and took two steps inside the 50-metre arc from about a 20-degree angle on the right forward flank.
[18][19] Overall, Kernahan played a total of 367 premiership games for Glenelg and Carlton, and kicked 1004 goals, which is the 13th most in elite Australian rules football.
[25] Following Graham Smorgon's failure to be re-elected at the 2007 Board Elections, Kernahan was made interim president, acquiring the services of and relinquishing the position to Richard Pratt within days.
[26] On 20 June 2008, he again took the role of president, this time permanently, after Pratt stood aside to fight charges of giving false and misleading evidence to an Australian Competition & Consumer Commission.