Northampton are currently top of the early Prem table after nine matches, and are hoping to repeat their title triumph of 2024.
“I’m incredibly grateful to Saints for the opportunity they gave me at a time when things were pretty uncertain in my career,” Millar Mills said.
“Walking through the doors here and being welcomed the way I was is something I’ll never forget.
“For now, my focus is fully on helping the lads finish this season as strongly as possible. I’m committed to giving everything for the group for the remainder of my time here.”
Graham’s four seasons at Franklin’s Gardens, meanwhile, also saw him handed the captaincy for a number of Premiership Rugby Cup matches.
“Coming in from a Champ club to an environment like this was a dream come true, and since then we’ve won a Prem title and created some amazing memories – being part of that group and that journey is something I’ll always treasure,” he said.
“I’ve loved my time at Saints, but it feels like the right moment to take the next step. That said, I still feel I’ve got plenty left to give.”
