- Colin Morgan as Merlin
- Angel Coulby as Gwen (Guinevere)
- Bradley James as Arthur Pendragon
- Katie McGrath as Morgana
- Antony Head as Uther Pendragon
- Richard Wilson as Gaius
- John Hurt as the voice of the Great Dragon
Recurring:
S01:
- Michelle Ryan as Nimueh
- Santiago Cabrera as Lancelot
- Caroline Faber as Hunith
- Asa Butterfield as Mordred
- Keith Durham as Tom the Blacksmith
- Michael Cronin as Geoffrey of Monmouth
- Rupert Young as Sir Leon
- Asa Butterfield as Mordred
- Santiago Cabrera as Lancelot
- Emilia Fox as Morgause
- Michael Cronin as Geoffrey of Monmouth
- Laura Donnely as Freya
S03:
- Rupert Young as Sir Leon
- Emilia Fox as Morgause
- Tom Ellis as King Cenred
- Eoin Macken as Gwaine
- Tom Hopper as Percival
- Santiago Cabrera as Lancelot
- Laura Donnely as Freya
- Adetomiwa Edun as Elyan
