The first King Tigers were seen by the enemy on 18 July 1944, thus deployed to Normandy a week earlier perhaps. The 503rd Heavy Panzer Batallion (s.H.Pz.Abt 503) were the first to have them. They were to hold back Operation Atlantic and fared well in the battle for Caen.
On the Eastern Front, the first King Tigers saw combat on 12 August 1944. Used by the 501st Heavy Panzer Battalion (s.H.Pz.Abt 501). They successfully ambushed up to a dozen T-34-85's.
Keep in mind that at these 2 dates very small numbers were in combat.