Mannion recently won the Colorado Classic stage race, where he took control on the stage 2 individual time trial up Vail Pass, and then held the lead over the following two stages to take the biggest win of his career.
The 27-year-old from Boston has ridden for UnitedHealthcare for the past two seasons, but despite the team's main sponsor pulling out after the 2018 season – which could mean that both the men's and women's teams fold if a replacement sponsor can't be found – Rally's interest means that Mannion will be safe to continue his upward trajectory at another US Pro Continental squad.
"Winning the Colorado Classic was definitely the biggest result of my career," said Mannion. "It came after a couple of really great and consistent seasons with UnitedHealthcare Pro Cycling, but I was always missing that big win. To be able to win the biggest race in my newly adopted home state was absolutely incredible."
Mannion began his career in 2010 with the Axel Merckx-run Trek-Livestrong squad, which has morphed into today's Hagens Berman Axeon team, and continues to serve as a launchpad for young riders into senior WorldTour and Pro Continental outfits.
"Rally Cycling is clearly one of the fastest-growing Pro Conti teams out there with a very good schedule," Mannion said. "I've actually been talking with Jonas [Carney, performance director] since 2009, back when he was directing Kelly Benefit Strategies. The programme has been taking some big steps these past couple of seasons, and the timing was just right for me to finally be able to come aboard.
"I think I bring consistency to the team and add another GC element to an already strong collection of GC riders," he added. "Being a 'younger' North American guy, I think my attitude will fit in nicely with the team, especially when we're racing in the gutters in Europe. Being alongside guys that you really get along with becomes hugely important."
Like Mannion, Carney was looking forward to seeing what the team's new signing will be able to bring to Rally's European sorties, in particular.
"Gavin has proven himself to be one of the very best in North America over the last couple of years," said Carney. "He's super-consistent, and still a young guy who is improving.
"We need more depth on our team as we transition to a heavier European schedule. Gavin is a logical pick as he checks all the boxes and will easily integrate with our existing riders."