Shiho Miyano and her older sister, Akemi, were born into the secret crime syndicate "The Black Organization". She was born to an English mother, Elena Miyano, and a Japanese father, Atsushi Miyano. Both of whom were members of the organization, and deceased. A prodigy, she was sent to the United States for education at a very young age, then later came back to Japan to finish her parents' secret project for the organization. Her nickname in the Organization is "Sherry", after the alcoholic drink.
Shiho was about 18 years old when she developed the experimental test compound for APTX 4869. She discovered that it could be used as a poison to kill without leaving any traces of itself in a body. In the manga, while researching its effects, Shiho noticed that a rare few of her lab mice had become younger after being given the drug instead of dying.
Meanwhile, the Black Organization started using APTX 4869 to commit untraceable murders. Unaware of the drug's rare side effect when they used it on Shinichi Kudo, the Organization orders a search for him because his body had not been found. Shiho searched Shinichi's house twice after he had been shrunk to confirm his death for the Black Organization. She began to suspect Shinichi's transformation after she searched his house for a second time and found that his clothes from primary school were missing. Shiho then confirmed him as deceased so the organization would not look into the matter further and kill him after all.
Her unhappiness over the Black Organization's usage of the APTX 4869 (for killing purposes), in addition to her open questioning over the murder of her sister, Akemi Miyano, resulted in her imprisonment and scheduled execution by the Black Organization. With no other way out, she took a pill of APTX 4869 to commit suicide.
Like Shinichi, she is one of the only two known rare cases who didn't die after taking the drug; but instead shrunk into a seven or eight year old girl. In her new smaller body, she escaped from the Black Organization and attempted to reach Shinichi's house, hoping he may be able to help her. Exhausted, she collapsed in front of Professor Agasa's house, who then found and adopted her