The Orioles selected Mayo's contract from Triple-A Norfolk on Friday.
The 22-year-old is joining Baltimore for his first look in the big leagues after Jordan Westburg suffered a fractured right hand Wednesday. Mayo will make his MLB debut Friday in Cleveland, starting at third base and batting eighth. He's one of the top prospects in baseball and has a .301/.375/.586 slash line with 20 homers in 77 games at Triple-A this season.