Rawlings, who played alongside Simpson for 10 years at North Melbourne, has spent the past two seasons as the Kangaroos' senior recruiter and induction coach.
He said North Melbourne had been a second family to him but he was looking forward to joining Simpson's team and making his mark at a new club.
"The role as assistant coach to 'Simmo' at the Eagles is a great opportunity and a challenge that I feel I am ready for," Rawlings said.
"I’ve always thought Adam was a senior coach in the making and I’m looking forward to working under someone I respect so highly.
"I would like to sincerely thank everyone at North for their support and for the understanding that has been extended to me throughout this process and decision."
Rawlings took up a position within the Kangaroos recruiting department before retiring at the end of 2011 and moved into the position of senior recruiter and induction coach.
A top-shelf, run-with midfielder and small defender later in his career, he played 245 games for North Melbourne, winning the best and fairest award three times, in 2004, 2006 and 2010 (equal).
Rawlings, who would have played a prominent role in the club's recruiting this year, leaves with the Kangaroos' blessings, according to coach Brad Scott.
"There’s no doubting the enormous contribution Brady has made to this football club," Scott said.
"He was a significant resource for our football and recruiting department with his knowledge and understanding of the game and players.
"We regard Brady as a true icon of our football club and we felt it was important to encourage him to explore this opportunity in order for him to further develop his skills.
"When Brady comes back to Melbourne, he knows the door here will always be open to him."
The Eagles are unlikely to replace the position of strategy and innovations coach vacated by Phillip Walsh, with Simpson to oversee a team of assistant coaches on the same level.
As well as Walsh, who returned to Port Adelaide, the Eagles have lost assistant coaches David Teague and Scott Burns.
With Simpson a first-time senior coach, it was considered likely the Eagles would look to replace Walsh in the senior strategy and innovations role.
However, football manager Craig Vozzo told AFL.com.au that was now unlikely.
The Eagles have appointed past players Adam Selwood and Jaymie Graham as development coaches.
Twitter: @AFL_Nathan
