[1] He is best known for portraying the character of Trystane Martell in the HBO series Game of Thrones, and Andrea Bocelli in the biopic The Music of Silence.

Sebastian was born in Oxford but lived in Andalusia, Spain, with his three sisters (all actresses): Florence, Arabella, and Rafaela for three years from 1999–2002.

He also appeared in The Red Tent as Re-mose, in Barely Lethal (alongside Hailee Steinfeld, Sophie Turner and Thomas Mann) as Cash Fenton.

"[citation needed] Sebastian portrayed Andrea Bocelli in the biopic The Music of Silence, alongside Antonio Banderas and Jordi Mollà directed by the Academy Award nominated director Michael Radford.

[5][6] In 2017, Sebastian starred in the feature film Trading Paint (also released as Burning Rubber) a sports film starring John Travolta, Michael Madsen and Shania Twain, directed by Karzan Kader.