The 24-year-old, a former Spanish Under-21 international who has since been capped by Equatorial Guinea, will become the club's fourth recruit of the summer when he completes the formalities of signing a three-year contract.
Emilio, who can play anywhere on the right, has played almost 200 games for Mallorca after starting out in the club's B Team. He also enjoyed successful loan spells with Castellon and Real Sociedad.
He was a member of the Spanish Under-19 side which won the European Championships in 2007, two years later he was part of the Mediterranean Games winning squad, and in 2011, he was a European Championship winner with Spain Under-21s.
Boro Head Coach Aitor Karanka said: "He is a versatile player who I have known for five years. He is a good lad with a good mentality and approach to the game.
"I know him personally and know the type of person he is, and that is important. And as a player he can play in different positions and I am delighted we have agreed a deal to sign him."
Emilio follows Kike, Tomas Mejias, and James Husband as the club's summer signings so far.