St Helens will replace one Thompson with another, with Manly veteran Joel Thompson set to join the Super League champions in 2021.
The Sea Eagles have granted Thompson a release from the final year of his Manly deal next season to take up a longer contract on offer in England.
St Helens have not made an official comment on Thompson's future but his looming move will go some way to offsetting the loss of Test prop Luke Thompson, who was reluctantly released to Canterbury six months early.
Joel Thompson's Super League shift comes after he almost made a similar move to Hull FC last year, before opting to stay at Manly on a reduced deal.
The 31-year-old has enjoyed a particularly strong run of form since Des Hasler's return to the Northern Beaches, playing 51 matches for the club to bring his career tally to 225.
He made his debut with Canberra in 2008, before stints with the Dragons and now Sea Eagles.
Earlier this year he captained the Indigenous All Stars in his seventh appearance for the side.
Thompson is also an influential figure off-the-field as an RLPA director and strong advocate for both Indigenous voices and mental health, founding The Mindset project in 2012 and being awarded the Ken Stephen Medal in 2016 for his community work.