The former Brisbane Lions star, who earlier this year hung up his boots after 218 AFL matches, will pack up his family and head south after Geelong announced it had lured the three-time premiership champ to Skilled Stadium as an assistant.
He will join Thompson and fellow deputies Ken Hinkley, Brendan McCartney, Brenton Sanderson and VFL coach Leigh Tudor.
Lappin said an initial chat with Cats footy manager Neil Balme planted the seeds of interest, before the former hard-running midfielder met some of the men he would be working alongside in the coaching box next season.
"I [then] met Mark at the airport about a week ago and things have just sort of evolved from there," Lappin told afl.com.au on Wednesday.
"They just really wanted to get to know me as a person. They obviously knew the way I went about things on and off the field as a footballer and they just wanted to see whether they thought I had anything to offer as a coach.
"I know I've got a hell of a lot to learn but I take down there a lot of acquired knowledge from playing in some good teams at Brisbane and playing under some good coaches and being involved with a good football club."
The day after the grand final loss to Hawthorn, Cats chief executive Brian Cook said many clubs – including Geelong – would emphasise development in coming seasons, with more coaches likely to be assigned the task of fast-tracking youngsters.
But Lappin, whose midfield experience might make him a prime candidate to look after that area, said he wasn't sure what role the Cats had penciled him in for.
"I'm not too sure of the general job description at this stage," he said.
"I think I'll sort of be helping out in a few areas and I'm looking forward to that."
New Lions coach Michael Voss had spoken to Lappin about the possibility his former teammate might change his mind about retirement.
But the 2004 best and fairest winner stayed firm, knowing his time was up. The only decision then was how, after 15 seasons at the Lions, he would spend the next chapter of his life.
"My wife and I entertained the thought of moving out of footy for a little while," Lappin said.
"Obviously the decision came down to me in the end, and once I'd spoken to Geelong I got really excited about the prospect of going and working with them.
"They're a fantastic club for a start … their playing list speaks for itself and their coaching staff are doing a hell of a lot of things right.
"The decision in the end was a pretty easy one."
