Lamborn was a valued member of the Stags squad in 2018 but was unable to return last season because of his commitments at the Rugby World Cup with the United States.
The 28-year-old has signed with the Blues for 2020 and has chosen to come back to the Stags to play his Mitre 10 Cup rugby.
He will bring strong leadership and plenty of energy.
Lamborn has captained the US team and captained the Stags in his last stint in Southland.
Coach Dale MacLeod said Lamborn would have an important role in the 2020 forward pack.
"I see Tony as a straight replacement for Brayden Mitchell who has been ruled out for the 2020 season," he said.
‘‘They both have very similar characteristics: tough; durable; competitive; experienced; perform at high levels consistently and will challenge everyone in the environment to make sure we get exciting performances on the pitch.
"I am looking forward to having him involved in 2020 and disappointed that Brayden won’t be.”
Mitchell, who played more than 50 games for the Southland Stags, is stepping down to give his body a rest. He has been one of Southland’s best players during the past decade.
There will be plenty of Stags players in action during the Super Rugby competition.
Ethan De Groot, Manaaki Selby-Rickitt and Ray Nu’u are with the Highlanders, with Flynn Thomas joining the franchise in the new year on a development contract.
Flynn is currently recovering from shoulder surgery.
Josh Moorby and Charles Alaimalo are part of a new development programme and pathway programme the Southland Stags have with the Highlanders.
Halfback Logan Crowley is with the Hurricanes.