He first appeared in the SGR chapter 48 (Incoming), wherein he was applying for a job at Percy Import/Export, alongside Eustace Boyce. (Ironically, he left with the erroneous assumption that The Boy is named "Terry" too, due to Hamilton Percy just calling him this, because he didn't like the name "Eustace".) It is later revealed that the job Mad Terry got consisted of licking labels at Percy's Peas.
In Bad Machinery (The Case Of The Team Spirit), the Mystery Boys also had to deal with Mad Terry; the boys make reference to a previous adventure they call The Case of Mad Terry, upon whose mention another boy will immediately demand that they change the subject. This case was so traumatic that the boys briefly swore off mysteries forever. Regardless, they are forced to interact with him again: In exchange for arranging for them a visit to Tackleford City FC's stadium, Lauren Field demanded that the boys shall bring her Mad Terry's trademark bow tie. After Linton Baxter deduced that he must in fact own more than that one bow tie, and must have several of them at home , Lauren barely managed to prevent Jack Finch from actually breaking into the house of Mad Terry and his Mum. (Whom everyone is calling Mrs. Terry , so that may actually be their surname.) Sonny Craven was the one who eventually got the bow tie instead , by convincing Bobby Swaps to help him in this matter. What exactly Swaps offered Terry in exchange for the tie remains a mystery.
At the time when Amy Beckwith-Chilton was about to give birth, the sports car of Shelley Winters proved to be inadequate for bringing her to the hospital through ice and snow. So they had to flag down another car, which happened to be the one of Mad Terry (whom Shelley prefers to call "Unconventional Terry") and his Mum. Even though Amy was initially reluctant to go with them, they arrived at the hospital safely and in time, and there Amy gave birth to a healthy litter  - er, to Walt Beckwith.
In order to protect her son from being bullied, Mrs. Terry at one point created a place for them that is secluded from the rest of Tackleford, populated by a handful of Tacklefordians who are "forgotten" in the sense that they had hardly appeared in any stories for a long time. In an attempt to prevent a mass escape of said Tacklefordians, Mrs. Terry accidentally injured her son in a quite severe way. Mad Terry's survival is uncertain.
One strange habit of Mad Terry was it to put a £5 note onto the pavement of a street, then to hide and to wait for someone to pick up the money, so that he can jump out and accuse the passerby of theft. Linton commented this with the notion that "everything Mad Terry does is a mystery".
It is unknown if there is any connection between Mad Terry and Terry Tibbs, the manager of the Tackleford municipal pool, appearing in the one-shot comic Human Soup, who, in addition to the name, is also somewhat physically resembling Mad Terry.