He is the writer of Thunderbirds: The Vault, and What Made Thunderbirds Go! - Anderson's second biography. Additionally he worked on Thunderbirds – A Complete Guide to the Classic Series as its editor, as well as contributing 'film guides' for the two feature films Thunderbirds Are Go and Thunderbird 6, and also supplying several interviews he conducted with the following: Gerry Anderson, Peter Briggs, David Lane, and Keith Shackleton. Furthermore Simon Archer, who conducted a interview with Paddy Seale that was recorded on tape, had this featured in the aforementioned book, with Hearn writing the interview up so it could be published.
- His legal first name is Simeon; Marcus is his middle name.