While McInnes's future has been the subject of much consternation over the past month, NRL.com understands the Sharks have snuck under their traditional rivals' nose with a lengthy deal worth around $500,000 a season for the St George Illawarra skipper.
No comment has been made by Cronulla or the Dragons at this stage.
McInnes informed teammates of his departure on Tuesday morning after negotiations with the Sharks began before Christmas.
Over that time speculation around his future under new coach Anthony Griffin mounted as the club kicked off negotiations with McInnes.
Throughout the talks, St George Illawarra officials stressed the 27-year-old was well and truly in their retention plans, but a formal offer was never pushed across the table.
Wests Tigers also had the NSW Origin squad member in their sights but never entered formal talks with McInnes after he met coach Michael Maguire, who he has a bond with from their South Sydney days, for a catch-up before Christmas.
McInnes is expected to play lock for the Sharks, a plan that is understood to have been communicated to rising young hooker Blayke Brailey.
His recruitment is a significant move for coach John Morris, the first major roster move in a year where several big-money contracts of star Sharks players come to an end.
McInnes is coming off the best season of his Telstra Premiership career and was selected in Brad Fittler's NSW squad for the State of Origin series but did not play in any of the three matches in November.
As for the Dragons, McInnes’s looming exit points to Andrew McCullough being pursued even more heavily from Brisbane.
The Broncos have insisted the veteran rake won’t be released, particularly with the Dragons pushing for Brisbane to pay a further portion of his 2021 deal.
New coach Kevin Walters has Jake Turpin on his roster while Kiwi international Issac Luke has remained at the club on a train-and-trial basis over the off-season.