In 1987, at age 10, Katori auditioned to enter Johnny & Associates, a Japanese talent agency that recruits and trains young boys, preteens to teens, to become singers and members of boy bands. After five of the other band members auditioning individually from 1986 through 1987, in autumn 1987, twenty boys, from ages ten to seventeen, were put together into a group called The Skate Boys, which was initially created as backup dancers for a famous boy band, Hikaru Genji. In April 1988, producer Johnny Kitagawa chose six out of the twenty boys to create a new boy band and named them "SMAP".
In 1988, he made his acting debut in a television series, Abunai Shonen III, along with his band members. After several supporting roles, in 1996, he landed his first major role in a drama television series, Tomei Ningen, which became a massive success. He has also starred in many high-rated television series, such as Hito ni Yasashiku (2002), Saiyuki (2006), and Bara no nai Hanaya (2008). In 2004, he starred in NHK's Taiga drama, Shinsengumi!. He is also a member of the Mitani Crew, a core group of actors that Japanese playwright Kōki Mitani tends to use in his work.
From October 1998, Nakai and Katori launched their own variety program, SataSma, which was created and aimed towards younger viewers and families. Katori starred in a segment titled, "Shingo Mama's Secret Breakfast", where Katori would cross-dress into a character called, "Shingo Mama", and visit houses to cook breakfast for children instead of their mother, allowing her to sleep in and rest in the morning. Katori started to used the phrase, "Ohha", a pun for "Ohayo", meaning "good morning" during the segment as his trademark phrase, which soon became a popular word used among children. Shingo Mama, the lovable character Katori played, and the phrase "Ohha", became a social phenomenon. As a result, SMAP incidentally brought in kindergartners and preteens to their fan base.
On August 16, 2000, Katori released a single, "Shingo Mama's Oha Rock", as Shingo Mama, which reached number one on the charts with over 1,200,000 physical copies sold and became a social phenomenon. In December 2000, the trademark phrase "Ohha", used by Katori, won the Ryukougo Taisho, an annual traditional event of announcing words that describes that year. From 2000-2001, Shingo Mama promoted the use of Ohha as part of a Ministry of Education campaign to encourage family communication.