The Saints confirmed Lyon's extraordinary homecoming in a media statement on Monday, under the headline of "He's back".
Club president Andrew Bassat told Saints members they started speaking to Lyon the day after previous coach Brett Ratten was sacked.
Lyon, Bassat and St Kilda CEO Simon Lethlean fronted a packed media conference at the Saints' Moorabbin headquarters, attended by the club's players, staff and new football manager Geoff Walsh.
The 55-year-old said he had unfinished business at the club after coaching them to within one point of their second premiership - in their 2010 grand final draw with Collingwood, which was followed by a 56-point defeat in the replay.
"Once this opportunity came up, I've reflected on the special bonds and friendships I have with past players and staff that I've maintained since 2007 to my departure (in 2011)," Lyon told reporters.
"When I presented to the board and how I felt about St Kilda, I did get very emotional and it sort of unleashed a lot of memories, the highs and the lows.
"That sort of really validated how I felt about the club."
On October 14, Bassat and Lethlean held a media conference to explain St Kilda's ruthless sacking of Lyon's predecessor Brett Ratten the night before.
That came less than 100 days after the club had handed Ratten a contract extension.
"We have spent the last 10 days talking extensively to Ross. He has presented to our Board and met multiple times with myself, Simon Lethlean and Geoff Walsh to share his vision for the future of our program," Bassat said.
Lyon will start officially on November 1, the same day Walsh also joins the club.
In his letter to members, Bassat said Lyon was appointed to develop a winning culture at the Saints.
"Ross' unwavering pursuit of on and off-field excellence is exactly what we believe we need to take this club forward," Bassat said.
"While his senior coaching record and reputation as a leader speaks for itself, Ross has a clear passion for St Kilda and is determined to bring the club and its people long-awaited success.
"Ross is aligned in our vision, and his experiences within the industry - particularly as someone with such in-depth knowledge of St Kilda - should give our members a high level of confidence moving forward."
Lyon is intent on building credibility for St Kilda, after twice leading them to losing grand finals during his previous five-year tenure.
The Saints are in the midst of the competition's longest premiership drought, dating back to their only VFL flag in 1966.
St Kilda's treatment of Ratten was labelled "disgraceful" and "demeaning" by former A-League coaching great Ernie Merrick, who spent most of the 2022 season as a part-time coaching mentor at the club.
Ratten's sacking was the result of a football department review during which Merrick backed the then-coach to keep his job.
Lyon was sacked as Fremantle coach in 2019.
Upon returning to St Kilda, Lyon becomes the first coach since David Parkin in 1991 - following a six-year break from Carlton - to have a second full-time stint in charge of an AFL club.
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