The Dockers announced the signing on Wednesday, with Boyd re-joining Longmuir after the pair worked alongside each other for two seasons with the Magpies.
Boyd, premiership winner and three-time All-Australian and best and fairest winner with the Bulldogs will work with the Fremantle defenders as an assistant coach.
Longmuir commended Boyd’s professionalism and welcomed him to the Dockers.
“Matthew is a person of really high character and I’m looking forward to seeing him get around our young players,” Longmuir said.
“I thought he was a really driven young coach when he was at Collingwood and he really helped me a lot. I also look forward to helping him develop his coaching career.
“He’s a good relationship builder. He’s very good at working with players one-on-one and he understands what it takes to be a professional footballer for a long time.
“He’s done a lot in footy, but most of all, he’s a good coach and I know he’ll impart a lot of his knowledge and experience on our players and make the team better.”
Boyd revealed Longmuir was a major factor in moving to Fremantle.
“I’m really excited to be a part of the culture that’s really building at Fremantle,” Boyd said.
“A big part of my decision to come over was clearly my relationship with Justin. He’s been an amazing mentor to me in the short coaching career that I’ve had so far and this was an opportunity to continue to learn from him.
“It’s an exciting time to be a part of the Fremantle Dockers after a really positive year and I know that the footy club is heading in the right direction.
“To be a part of that at the start of a really exciting journey was something that appealed to me.”
Boyd won the 2016 premiership with Dockers defender Joel Hamling and touched on the exciting prospect of joining a youthful coaching group.
“I think footy is the hardest game in the world and your ability to empathise and relate to what the players are going through is important,” Boyd said.
“It’s about being able to teach and educate so the right message is getting through and the players are understanding as best as possible.
“While it’s obviously great to have experience, youth brings a whole lot of energy and vitality for the job and I’ll certainly bring my energy to the role as best as I can.
“I’m really excited to work with the young and emerging backline and I can’t wait to get over there and meet the boys and try and help them as best as I can.”