Frank, who made the top ten in the Tour de France in 2015, has also ridden for BMC Racing in the past an moves to a French team for the first time in his career.
At AG2R he will look to support GC leader Romain Bardet in the Tour de France and several other stage races, while also having the chance to lead the squad when the opportunity arises.
"For the first time in my career I will ride for a French cycling team. Then, being a teammate is a role that suits me well, especially when you have to assist a great champion like Romain Bardet. I was already in that position with Tejay van Garderen and I really liked it," he said in a press release issued by his new team.
"Obviously, I also have personal ambitions and I want to win races for my new team."
Frank, 29, abandoned this year's Tour de France and struggled at the Tour de Suisse in the build-up, abandoning his home stage race too. He finished in the top ten at the Tour of Romandie earlier in the year.
"Mathias is a great rider specialized in long-term races. His task will be to help Romain Bardet in the mountains but he will also share leadership for some races," said team boss Vincent Lavenu.
AG2R have been busy in the transfer market this summer having brought in Rudy Barbier (Roubaix Metropole), Oliver Naesen (IAM Cycling) and Stijn Vandenbergh (Etixx-Quick Step). They have also extended contracts with Mikaël Cherel, Hubert Dupont, Axel Domont and Jan Bakelants. The team also recently confirmed that their title sponsorship had been extended.