Playing for the US at the World Cup in Tokyo will be his priority next year.
A 2012 North Otago representative who has played 16 matches for the US since his international debut in 2016, Lamborn, was offered a Magpies contract earlier this year but turned it down because he had options in Europe to consider.
"When mum got quite sick with her breast cancer I decided to stay in New Zealand for another year to be closer to her," Lamborn said referring to his mother Sari.
"By then it was too late for Hawke's Bay and I had to look for other options."
Ironically the Magpies will open their Mitre 10 Cup Championship campaign against the Stags in Invercargill on August 19.
"I'll be giving the Stags 100 per cent. I'll be trying as hard as I can to give them the win," Lamborn, a four-game Hurricane in 2016, said.
Magpies head coach Mark Ozich described missing out on Lamborn's services as "one of those unfortunate situations."
"By the time Tony decided to come back to us there was no space on our roster. We had no money for an extra loosie. If we contracted Tony we would have had to go with one less front rower or one less lock."
"Everything is amicable and he could return to the Magpies in the future. That's the nature of the beast when it comes to professional rugby," Ozich added.
The president of Lamborn's Havelock North club, Conrad Waitoa, said he hoped Lamborn would return to the Magpies nest when his World Cup commitments finished.
"This is a massive loss to rugby in Hawke's Bay. Tony brings more than just his on-field abilities. Off the field, he'd always put others first, he would give the shirt off his back and would bring his gear, shorts and boots to the kids and club players. Tony would always help coach at the lesser high schools and share his enthusiasm and passion with those high school players and coaches.
"He's always down at our village junior games and first to help around the club," Waitoa added.
