The answer to this question depends upon if one believes King Arthur was a fictional character or not. Historians have argued, over many generations, if Arthur was real. Therefore, Arthur's origin depends upon if one believes in the fictional Arthur or the nonfictional Arthur.
If one only believes in the fictional Arthur, then he reigned as king of Camelot. According to literary and historical critics, Camelot was located somewhere in Britain (exact location is unknown). Other critics believe that Camelot is completely fictional and cannot be aligned with any geographical location.
On the other hand, if one believes that Arthur was real, then he most likely would have been from England (given some "Tudor monarchs traced their lineage to Arthur and used that connection as a justification for their reign").
King Arthur was from Camelot.
If you are talking about the fictional character, he was from the fictional kingdom of Camelot. If you are talking about the person he was supposed to be based off of, no one really knows. Some people say that he is based off of a Roman general. Some people say that if he was real, he would have been a king from England. Some even say that he will come again when the people of England need him!
King Arthur is believed to be a mythical character who was the leader of the Kingdom of Camelot and the Knights of the Round Table. Many authors describe him as a warrior more than a king because he led his army to wage war against Saxons and won twelve times against them. The historical Welsh tale Culhwch describes him as a leader of mythological warriors who had super powers. If Arthur really existed then he most probably was a warrior in the 5th century.