Longmire departs Sydney after 25 years in various roles to reunite with his former assistant, new Bombers coach Mark McVeigh, in a new role.
"I'm really excited to be joining the Essendon Football Club," Longmire said in a statement.
"I've known Mark for a long time and have worked with him as well. He is passionate about the Bombers and has a clear vision for the team.
"I look forward to supporting both him and the staff as the club looks to take the necessary steps back towards being a consistent team who challenge the opposition every week."
The Bombers have effectively split the football boss role in two, with Longmire to oversee game strategy, player development, leadership, the coaching group, performance analytics and strength and conditioning.
The 55-year-old, most recently Sydney's executive director of club performance, will start this week ahead of the trade period.
Landing Longmire, who didn't pursue any vacant head coach roles, is a huge result for the battling Bombers, who finished last on the ladder and needed to put support around first-year coach McVeigh.
"John's football experience and leadership are exactly what we have been looking for in this role," Essendon chief executive Tim Roberts said in a statement.
"We have made no secret of our ambition to build a football program capable of delivering sustained success, and bringing in someone of John's calibre is a significant investment in that goal.
"Over 14 seasons as senior coach of the Sydney Swans, John built a program that consistently competed at the pointy end, including the 2012 premiership and four further grand final appearances.
"He knows what a winning program looks like from the inside and how to sustain it year after year.
"Just as importantly, John will be an enormous asset to Mark McVeigh.
"Very few people understand the demands of the senior coach role like John does and having someone of his experience alongside Mark to provide support, guidance and a sounding board will allow Mark to focus on coaching and developing our players."
Swans chairman Andrew Pridham hailed Longmire as a "much-loved and respected figure" and "legend".
Longmire joined Sydney as an assistant coach in 2002 and was involved in the drought-breaking 2005 premiership, later succeeding Paul Roos.
Across 333 games, Longmire, the Swans' longest-serving coach had a 63 per cent winning record, won the 2012 premiership and led Sydney to five grand finals and 12 finals campaigns.
He stepped down after the 2024 grand final loss to Brisbane.
"Having arrived at the Swans 25 years ago this club has meant so much to me and my family," Longmire said in a Swans statement.
"I will look back with extreme fondness of my time in Sydney and always be proud of what we achieved as a football club."
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