Since King Arthur is a bard (albeit a frivolous one) in Welsh Myth, that technically means Arthur is a magician in some capacity.
Bards/Poets in Celtic Culture and Myth are combo of newscaster, historian, genealogist, prophet and entertainer. They are essentially lore keepers - Walking libraries for a mostly illiterate society. Which is why they are placed in an esteemed social status. They not only entertain but can prophesize the future, rouse one's companions to action and curse people with satires, on top of preserving knowledge through poetic forms.
Taliesin and Myrddin Wyllt are examples of from welsh lore. In Irish myth, there's Laidcenn, Niall (of the nine hostages)'s poet, and I remember some poet-warrior threatened to lampoon Cu Chulainn.
I think evidence that points to Arthur's magic is in the story Gwyn, Gwythyr and Creiddylad:
If you interpret Gwyn and Gwythyr as mortal men, Arthur magically oath-bound two men to fight each other until judgement day, granting them immortality in the process.
If you interpret Gwyn and Gwythyr are divine beings, then Arthur cursed two gods to fight each other annually and organized the seasons.
No matter how you cut it, Arthur is pretty magically strong. Other magical facts about Arthur include:
Welsh Triads saying Arthur had the ability to render the land infertile for seven years wherever he walks.
Arthur having an invisibility cloak, like Harry Potter. It was called "Gwenn", apparently.
Oral Folklore that says that Arthur can shapeshift into a raven or chough. Which is why it's bad luck to kill one.
Uther, Arthur's dad, being the creator of one of the "Three great enchantments of Britain", which was then taught to Menw ap Teirgwaedd, an enchanter knight of Arthur's. In his death-song, Uther even boasts about being a "great enchanter". (unless I'm mistaken)
