Guillermo began acting at the age of 11. He was born in Bilbao to an English father and Spanish/Irish mother, and lives in London. He has worked extensively in theatre and film. His first experience was in a double header with Dudley Sutton for Art Angel, directed by Lu Kemp, in the experimental 'Have Your Circumstances Changed?' His theatre debut was in director Rupert Goold's 'Medea' at The Almeida Theatre. Since then he has played the title roles of Charlie in the Sam Mendes directed 'Charlie and the Chocolate Factory' at the Theatre Royal Drury Lane, Adrian Mole in Luke Shepard's 'The Diary of Adrian Mole' at the Menier Chocolate Factory, and Young Louis Braille in 'The Braille Legacy' at the Charing Cross Theatre directed by Tom Southerland. In 2018 he played the title role of Alexander, at The Old Vic, in a new adaptation of 'Fanny and Alexander' by Stephen Beresford, directed by Max Webster.

In film Guillermo has played the role of Hilary Tolkien in 'Tolkien' directed by Dome Karukoski, the role of Geoff Dwight in 'Rocketman' directed by Dexter Fletcher, furthermore he has lead the film 'Animal/Humano' as the lead character 'Matteo' for Pecado Films ,written and directed by Alessandro Pugno.

For Television he has played the role of Seb in the Rufus Jones' series 'Home' for Channel X, he played Alfred in 'Bleak House Guest' directed by Robert Delamere. He has also completed a Christmas special podcast for Story Glass/Freemantle' Baby it's Cold Outside' directed by Virginia Gilbert. He's played Eric in' Vis A Vis El Oasis' for Globomedia and Netflix, he has played Tom in season one and two of 'The Outlaws' for the BBC and Amazon Prime alongside Chirstopher Walken and Stephen Merchant, and he recently played Fernando in "perverso" produced by Amazon prime and Onza, directed by Gustavo Ron and Federico Untermann.