The former North Melbourne senior coach and Port Adelaide assistant departed this week after joining the club for the 2012 season.
At Carlton, Laidley will reunite with former coach and mentor Mick Malthouse.
Blues football manager Andrew McKay said Laidley was a terrific addition to Carlton’s coaching ranks.
"Dean has vast football experience as both a player and coach, and we believe he is the right man to take Carlton's midfield forward over the coming years," he said.
Laidley joins a raft of officials to leave the Saints in recent times, with assistant coach Jaymie Graham and high performance manager Bill Davoren both taking roles elsewhere.
Graham has accepted a development role with West Coast while Davoren will head up Collingwood's fitness department after the departure of David Buttifant.
Laidley was in charge of the Saints' midfield, with the club now set to reshuffle coaching roles.
It is believed other assistants Adam Kingsley, Tony Micale and Danny Sexton will remain at the club.
The Saints are also expected to announce a new appointment to their coaching staff in the coming days.
St Kilda head of football Chris Pelchen thanked Laidley for his time with the Saints.
"Dean has made a valuable contribution to the Saints over the past two years," Pelchen said.
"We wish him well for his future."
There have been a number of changes to St Kilda's structure over the past few months amid reported tension between coach Scott Watters and Pelchen.
CEO Michael Nettlefold resigned from his post in late September and will leave the club once an appointment is made to replace him.
Football manager Greg Hutchison was made redundant during the finals, with his duties redistributed through the football department run by Pelchen.
President Greg Westaway has been replaced by AVJennings boss and club board member Peter Summers after serving his maximum term of six years.
The Saints were also forced to deny Watters had been sacked earlier this month after former club president Andrew Plympton suggested it on stage at a post-finals function.