Yoruichi grew up in Soul Society with her friends, Kisuke Urahara, as he, along with Tessai Tsukabishi, lived at her manor and they played together daily under the Sōkyoku hill training space, which Urahara built when they were children.
Yoruichi was the once the 2nd Division Captain of the Gotei 13, Commander of the Onmitsukidō, and Corps Commander of the Corrections Corps. She was also the 22nd generational head of the Shihōin family , one of the four noble families, and the first female to hold that position. She earned much respect in Soul Society in all of her positions. While her age has not been revealed, she is considerably older than Byakuya Kuchiki and refers to him as "Little Byakuya" since he was a kid. She used to tease him and they used to play tag using flash steps, even when Byakuya was still a kid and she was already a captain. She was probably on good terms with his grandfather considering they both were heads of two of the most powerful noble families.
While Yoruichi was the Onmitsukidō commander, she took in Soifon, recognizing her talent, as a guard and became her mentor and friend. She taught Soifon most of the techniques she knew. As a result, Soifon succeeded her as head of the Onmitsukidō after Yoruichi's escape from Soul Society. During her time in Soul Society, she created many techniques that involve Flash Steps, as well as teaching some of them to Byakuya.About 110 years prior to the current time, Yoruichi had recommended that Kisuke Urahara, then her 3rd Seat, to be appointed to the recently opened position of Captain of the 12th Division, after the former Captain Kirio Hikifune was promoted. She constantly coached Urahara in the best way to be assertive and confident in his new role. Her time as a Captain had shown her personality to be the same as it is currently, she is wise and serious as a leader but beyond that she is laid back and playful. Yoruichi was noticeably unperturbed about her former 3rd seated officer and current fellow captain waltzing into the Maggot's Nest despite needing proper permission, an attitude which Soifon appeared to reprimand her for, because she could not remember if she gave it. She also spent her time teasing the young Byakuya, stealing his ribbon at one time to entice him into another game of tag, to which he would call her "Were-Cat".
When Kisuke Urahara and Tessai Tsukabishi were being trialed and sentenced for the Hollowfication of other Shinigami and the use of forbidden Kidō, Yoruichi appeared as an unseen figure, helping them escape from their trial to an underground training room beneath Sōkyoku Hill, where she had already brought all of the Vizards that were sentenced to be exterminated by the Shinigami.
She revealed to Urahara that she also brought his new Gigai and ordered him to make 10 for himself, Tessai and the Vizards. After wards she decided it would be best to disappear, as it would only be a matter of time before the Shinigami realized the rescue was her doing. When she left Soul Society, she left behind her titles and positions and was thus relieved of her high status. She was succeeded as Captain and Commander by her protégé, Soifon, at a later date.
As a former captain of the Gotei 13, it is known that Yoruichi possesses a Zanpakutō and has achieved Bankai, but she relies nearly exclusively on hand-to-hand combat, Flash Cry and Flash Steps. Although she never carries it during the main storyline, it is shown during a flashback about how Soifon became Yoruichi's protégé and when she tells Ichigo about Kisuke Urahara's role as previous captain of the 12th division; It resembles a kodachi or wakizashi.It was kept sheathed horizontally behind Yoruichi's lower back. She briefly uses a sword of said size during the The Sealed Sword Frenzy OVA to dispatch a Hollow, though it is unknown if this was her Zanpakutō.