The Spain U21 defender moved to the Gunners on Thursday from Valencia.
Mosquera joins for a €15m fee and has penned a deal to 2030 with an option for another 12 months.
“It means a lot to me,” the 21 year-old told arsenal.com. “There’s been a lot of hard work behind all of this.
“It’s an opportunity that came up and I couldn’t let it pass me by. I’m joining a historic, massive club. When you arrive here with your family, you really feel the greatness of this place.”
Mosquera added, “It’s true my career has moved faster than expected and I’ve had to mature fast. Coming here is a chance to keep learning and improving. I’ll be working with great players and a top-class coaching staff, so that will help me develop personally.
“I’m a player with a lot of energy. I’m young, so I still have plenty to give. I’m fully motivated and excited. Above all, I want to improve, which will create competition within the squad. I’m really happy.”