McCormack will go straight into the squad to face Watford today and manager Grayson said: "Ross is a player of good quality. He has proved in the Championship that he can score goals and, hopefully, he will continue to do that with us."
McCormack, also a transfer target for Hull City, scored 33 goals in 88 appearances for Cardiff after moving to Wales from Motherwell two years ago.
He is leaving the Bluebirds after falling further down the pecking order of strikers since Craig Bellamy's shock move to his home-town club from Manchester City.
John Eustace, who Leeds came very close to signing earlier this year, should start for Watford.