Perhaps most well known for his portrayal of Marcus Flint in the 2001 feature film Harry Potter and the Sorcerer's Stone, he is also notable for his role as Roddy Oliver in the children's television series The Ghost Hunter.
He made his screen debut in the 1999 short film Killing Joe.
In 2002, he appeared in the children's series Sir Gadabout, the Worst Knight in the Land.
Though his personal life is little publicized, Theakston has a Ghost Hunter television "family" that includes actors Verity-Jane Dearsley (as Tessa Oliver), Tracy Brabin (as Mrs. Oliver), and John McAndrew (as Mr. Oliver).
In the first Harry Potter film, Theakston and Jamie Waylett both play members of the Slytherin House.