Thomas came from a family of Sighted mundanes who has served Shadowhunters for centuries. His parents once served the residents of the London Institute, particularly the parents of the people he came to serve.
He was a small and frail twelve-year-old boy when he was sent to the Institute. Being the only boys in the Institute of the same age, he and Will Herondale became sort of friends until Jem Carstairs arrived, after which Will's attention turned almost completely to the latter. Even so, Thomas continued to address Will in a friendly manner, albeit with a small amount of sadness.
Thomas grew to be a strong, muscular man, bearing a commanding physical presence that developed over the years of serving and training with the Nephilim. He also fell in love with the ladies' maid and parlor girl, Sophie Collins.
Along with Henry Branwell and Will, Thomas went to the Dark House on a scouting mission which later turned into a rescue operation. He went into combat with the two Shadowhunters and helped save Tessa Gray from the Dark Sisters. During Tessa's stay at the Institute, she met Thomas and, as she was quite close to Sophie, quickly suspected Thomas's feelings for the parlor maid.
During the Magister's attack on the Institute, Thomas fought the invading automatons, instructing the remaining women to hide in the Sanctuary. Even before Jessamine left, Thomas was already outnumbered, and he was eventually overpowered and defeated. Will later found him at the brink of death and would have given him the Shadowhunter farewell if Thomas hadn't stopped him. Thomas, with the last bits of his strength, then told Will that Tessa was alive, and that he take care of Sophie.
For his bravery, Thomas's ashes were spread through the Silent City, an honor very rarely given to non-Shadowhunters. He was replaced as the carriage driver of the Institute by his brother, Cyril, whose presence and sheer resemblance to Thomas sorely reminded the residents of the Institute of him. Years later, Sophie named her only son after him.
Thomas was a tall and muscular young man, with dark brown hair and brown eyes.
- Clockwork Angel (first appearance)
- Clockwork Prince (mentioned only)
- Clockwork Prince: Manga (mentioned only)
- Chain of Gold (mentioned only)
- Thomas sometimes spoke with an East End London accent.