Ratten has joined the Kangaroos on a part-time basis and will work with their midfield group, supporting fellow assistant Jordan Russell.
He previously worked under new North Melbourne boss Clarkson for six seasons between 2013 and 2018, including playing a role in Hawthorn's premiership three-peat between 2013 and 2015.
Ratten was sacked by St Kilda on October 13, less than 100 days after signing a two-year contract extension, and has since been replaced by Ross Lyon.
The 51-year-old was St Kilda's caretaker coach for the final six rounds of the 2019 season, following Alan Richardson's departure, then was senior coach from 2020 until 2022.
Ratten also coached Carlton between 2007 and 2012.
"Brett comes to the club as an experienced, knowledgeable football person and we are thrilled to welcome him to Arden St," North Melbourne's general manager of football Todd Viney said in a statement.
"Brett is a highly regarded operator who has been part of premiership success during his time at Hawthorn.
"His vast coaching experience will be a huge asset for our club."
The Kangaroos said they would look to fill a remaining coaching vacancy in the coming weeks, after not renewing the contracts of former assistants Anthony Rocca and Heath Younie.
Clarkson started his job at North on November 2 despite a panel investigating serious allegations of racism at Hawthorn between 2008 and 2016.
Clarkson and Brisbane Lions coach Chris Fagan are key figures in the independent investigation.
Both men strongly deny all claims made against them.
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