Kogoro is a private investigator, but he prefers to laze around his office all day unless his client is a beautiful woman. Kogoro attended Teitan High School, the same high school as his daughter Ran Mouri and Shinichi Kudo , along with Eri Kisaki _(his future wife)_, Yukiko Fujimine, and through kindergarten to middle school attended the same schools as the actress Ruri Ujo and Eri. He then graduated from Beika University, where he was the ace of the judo team, though he was highly vulnerable to stage fright.

Some time after graduating, Kogoro became a police officer, working in the Tokyo Metropolitan Police Division One with Inspector Megure, but while an excellent marksman he was not a particularly good investigator. According to Ninzaburo Shiratori in Movie 2, however, the duo were famous for the way they apprehended criminals. He also worked for some time under Inspector Yuminaga in the arson division. While Movie 2 strongly suggests that Kogoro left the police force because of the incident where he gave his wife, Eri Kisaki, a bullet graze on the leg to save her from a hostage situation, his reasons for leaving the force have not been confirmed in the manga. Around the time Kogoro left the police force, Eri also moved out, though they have never officially divorced despite ten years having passed. Their daughter, Ran, keeps trying to get them back together, but she has yet to succeed. Despite constantly feigning disinterest in her, Kogoro still has feelings for Eri. He even asked her to move back in with him at one point, but at the time she pretended not to hear him because she was not "ready" yet, though she still has feelings for him as well. These are expressed mainly through jealousy when Kogoro's attention is caught by other attractive women in the vicinity, which occurs on a regular basis.

After Shinichi was forced to adopt the identity of Conan Edogawa, Hiroshi Agasa concluded that Shinichi couldn't openly continue investigating the Black Organization without attracting undue attention. To maintain his cover, Agasa suggested Shinichi to move in with Kogoro and use him as a front. During a case Kogoro took soon after, he reached the wrong conclusion, and Conan knocked him out with an ashtray before he could accuse an innocent person. Kogoro conveniently falls unconscious in a reclining pose, and Conan uses his Voice Changer to imitate Kogoro's voice, presenting his conclusions and the identity of the true murderer. After a few more cases occur in this fashion _(though Conan uses his Stun-Gun Wristwatch instead of an ashtray)_, Kogoro receives the title "Sleeping Kogoro" and a public reputation as a master detective. In the few instances where Kogoro remains awake for some reason, Conan still does the main detective work and uses Kogoro as his official mouth-piece by drawing Kogoro's attentions to important clues he has uncovered.

(Source: Detective Conan World)