Cogman worked on the Game of Thrones pilot as a writing assistant and on the full series as a script-editor and writer. He has also written three episodes of the series, as well as many of the "history" special features on the Season 1 DVD/Blu-ray set. He also serves as the unofficial "keeper of the mythos" for the show. He also wrote the series bible outlining character and background information for other writers, actors and crew to consult. He is the author of Inside HBO's Game of Thrones, the official companion guide to the series.
Cogman will return to write two episodes of Season 5. In June 2014 it was announced that Cogman will also be the scriptwriter for a movie adaptation of the Magic: The Gathering fantasy card-game series.
Cogman also wrote the hour-long series of animated featurettes included in the TV season Blu ray set for Season 1 (narrated by the TV series' starring cast, in-character), known as the "Complete Guide to Westeros". Although Dave Hill took over the writing responsibilities for the new format known as the "Histories & Lore" series, Cogman still oversaw it.
The sigil next to Cogman's name in the opening credits is actually a vomiting maester - a joke by Benioff and Weiss, referencing how at a certain season wrap party, Cogman had too much to drink and had to throw up.