He is played by the late Godfrey Gao in the first book's film adaption The Mortal Instruments: City of Bones.
In the TV adaptation, Shadowhunter, he is played by Harry Shum Jr.
Magnus was born to a half-Dutch/half-Indonesian woman and an unknown demon in 16th century Indonesia and due to being half-demon he like other Warlock's had a mark, his was cat-like eyes.
After discovering this Magnus's mother hung herself and his stepfather tried to drown him but Magnus saved himself by using his magic to burn his stepfather. After this he was brought up by churchmen and raised by the Silent Brothers of Madrid.
Every two years until 2007 Magnus would cast a spell on Clary Fray to remove her memories of seeing anything from the Shadow World on her mother Jocelyn's wishes as she wanted to protect her daughter from it and from Clary's father Valentine.