His role will be finalised in the coming month before starting in November.
Adcock was delisted by Justin Leppitsch at the end of 2015 and picked up in the rookie draft by the Western Bulldogs.
The 30-year-old played seven games for the Dogs this season and was part of the Footscray VFL team that won the flag.
He told lions.com.au his experience with the premiers taught him a lot.
“I’m really, really excited to be back,” said Adcock.
“The year away has done a lot of good for me – I learnt a lot about myself and a lot about footy and where the game is at now. Even though I didn’t play in the game (the Grand Final), it was still great to be part of the club.
“Hopefully I can pass on some of that knowledge and the learnings from that to support the club.”
Fagan had met with Adcock, who is doing a coaching course in America, in Melbourne to discuss the role.
“We’re really pleased to welcome Jed back to the Lions,” Fagan told the club website.
“As a former club captain, he knows what it takes to mentor a group of young guys.
“After a year away at the Western Bulldogs, and with a VFL Premiership under his belt, Jed will only bolster our coaching group.”
Adcock announced in August he would retire at the end of the 2016 season.