As a child, Amadeus Arkham lived with his mother, Elizabeth Arkham, in a family mansion. Elizabeth was very sick and suffered from debilitating mental illness. It was revealed in Amadeus Diary that in 1920 he actually killed his mother releasing her from her madness, cutting her throat. Showing the beginning of his insanity. It was also hinted that his mother sexually abused him as a child. He would go on to repress this memory and his murder, instead believing that she had committed suicide. Amadeus was the sole heir to the estate and would inherit it in his mother's will.
Because of his mother's illness, Amadeus gained a growing interest in the study of mental illness, and decided to treat the mentally ill in hopes of avoiding going through what his mother went through. Arkham married a woman named Constance and together they had a daughter, Harriet. Arkham worked at the state psychiatric hospital in Metropolis, and lived there with his family. One day Martin “Mad Dog” Hawkins was brought to him for an interview. He found Hawkins to, indeed, be mentally ill. This prompted Arkham to begin to think about all of the criminals inside the system who were there simply because they were sick. He began to remodel his mansion to facilitate the treatment of the mentally ill. Eventually, Arkham told his family about his plans with the family estate, and they moved into the estate to oversee the renovation from the mansion to an asylum.
One evening in 1921 Arkham received a phone call about the escape of Mad Dog Hawkins. The police sought his expert opinion to help aid in Hawkins's capture. Arkham gave them a brief answer, but paid them little mind. Soon after when returning from an errand, Arkham found his wife and child brutally raped and murdered, with his daughter's head severed and placed in her doll house. This was the work of Hawkins.
Arkham continued the renovation, and later saw the opening of the Elizabeth Arkham Asylum, naming it after his mother. Mad Dog Hawkins would be the Asylum's first patient. Arkham was praised for his compassion and care for taking in Hawkins despite what had happened to his family. Later Arkham would murder Hawkins during electroshock therapy, and the incident would be labeled an accident.
Amadues began to fall into insanity. He became extremely paranoid, going as far as to tape over all of the mirrors in the mansion. In one Arkham riot Dr Cavendish, a psychiatrist of the institution, told Batman that Amadeus tried to kill his stockbroker in 1929, which led him to his own sanatorium as a patient. Through the years Amadeus read about shamanic practices and magical rituals in his confinement. He used his fingernails for writting circular spells in the cell floor until his death, where he was found by a nurse.
DC's New 52 Relaunch
Amadues was brought in as a feature character in the rebooted All-Star Western, partnering him with the bounty hunter Jonah Hex.
In the new storyline, which primarily takes place in Gotham City during the 1880s, Arkham is a psychologist who specializes in criminal behavior. He lives in a mansion in Gotham with his mother. He initially partners with Hex in an effort to solve a series of murders that are taking place in the city by a suspect the police refer to as "The Gotham Butcher." Through the ensuing issues, the unlikely pair continue to work with another to solve a series of crimes and cases in Gotham, as well as in New Orleans.
Amadeus is mentioned several times in the 2009 game Batman: Arkham Asylum. Throughout the game, the player can collect journals from the "Spirit of Arkham" that claims to be possessed by the spirit of Amadeus. The first ten or so recount Amadeus' history up to his murder of Hawkins. However, the spirit later turns out to be the current warden of the Asylum, Quincey Sharpe.
Amadeus' cell appears in the basement of the asylum as well, with his spells still carved into the floor and walls. Scanning the cell unlocks a bio of Amadeus. A plaque in the botanical garden is dedicated to Amadeus' wife Constance, but has been defaced with the phrase MAD DOG carved into it. The statue above the plaque has been decapitated, just like Amadeus' daughter.