Prior to 1969, Adachi began submitting works to the manga magazine COM. In 1969, he followed his older brother's lead and moved to Tokyo to begin work as an assistant to mangaka Isami Ishii The following year, he made his professional debut with Kieta Bakuon. He continued publishing various short stories and shorter series throughout the 1970s based on works created by others, the most well known being his adaptation of Rainbowman from 1972-1973. In 1978, he published his first original series, Nine, in Weekly Shōnen Sunday. He published two other original series, Hiatari Ryōkō! from 1979 to 1981 in Weekly Shōjo Comic, and Miyuki from 1980-1984 in Shōnen Big Comic.
Adachi became a household name with the publication of his series Touch from 1981 to 1986 in Weekly Shōnen Sunday. In 1982, Hiatari Ryōkō! was adapted into a live action TV drama series. The following year, 1983, was a big year for Adachi. He received the 28th Annual Shogakukan Manga Award for shōnen/shōjo manga his two series Touch and Miyuki. His Miyuki series was adapted into both a TV anime series and a live-action film, and Nine was adapted into three films, with another following in 1984.
Touch was adapted into a TV anime series in 1985, and the series ran for two years on Fuji TV. Adachi's romantic shōjo manga series, Slow Step was serialized in Ciao from 1986 to 1991, and another romantic comedy series, Rough, appeared in Weekly Shōnen Sunday from 1987 to 1989. Adachi then released Niji Iro Tōgarashi, a fantasy medieval romantic comedy manga series, from 1990 to 1992 in Weekly Shōnen Sunday.
Jinbē, a romantic comedy about the relationship between a stepfather and stepdaughter, was serialized in Big Comic Original from 1992 to 1997. Adachi's longest manga series, H2 was serialized in Weekly Shōnen Sunday from 1992 to 1999 and compiled in 34 volumes. This manga was adapted into a TV anime series which aired on TV Asahi from 1995 to 1996.
From 2000 to 2001, Adachi published a fantasy romantic comedy series in Weekly Shōnen Sunday titled Itsumo Misora. His next longer series was the boxing romantic comedy, Katsu!, published from 2001 to 2005 in Weekly Shōnen Sunday. In 2005, H2 was adapted into a live action drama series aired on TBS in Japan, and Touch was adapted into a live action movie released by Toho. He also began his manga series Cross Game, serialized in Weekly Shōnen Sunday. The following year, Rough was adapted into a live action movie, also released by Toho.
Due to achieving total manga sales numbering over 200 million volumes, Weekly Shōnen Sunday devoted issue 26 in 2008 to Adachi and his works. In 2009, Adachi won the 54th Annual Shogakukan Manga Award for shōnen manga for Cross Game, which was adapted into a TV anime series which began airing on TV Tokyo in April 2009.
Adachi began a new manga series, Q and A, in the inaugural issue of Monthly Shōnen Sunday in June 2009.