McCarthy, who turns 22 today (September 8) joined the team as a neo-pro in 2013 and has impressed team manager Bjarne Riis in the two-seasons that he has ridden for the team.
"For me it was obvious Jay McCarthy should continue," Riis said on the team website. "He proved in the last week of the Giro d'Italia that he could achieve the level we had signed him for a couple of years ago. He had a number of very good stages in the Giro, and after that he went on performing well."
McCarthy made his grand tour debut at the 2014 Giro and came close to victory on stage 17 to Vittorio Veneto when he finished in third place.
"He is still a very young guy and has to improve on many things but he now has a chance to continue next year and work on those areas," Riis added. "I look forward to following his progress."
Pleased with his extension, McCarthy said he was looking forward to finishing the season strongly and repaying the team's faith.
"I'm, of course, very excited about continuing next year," he said. "I feel that I've had a good development during the last two seasons and I learned a lot from racing with my teammates as well as receiving great support and guidance from the team's staff.
"We're going to have a very strong team in 2015, also with the additional signings, and I look forward to helping out as much as possible," he said. "I need to continue to work hard and dedicate myself fully but it motivates me a lot being on a very aspiring team. Now I focus on performing well for the team before preparing myself for the next season."