Koreyoshi was said to be intelligent and wise from a young age, reading books and composing poetry before Emperor Saga from the age of 11.
In 835 he became a top-ranked student of Chinese literature in the Daigaku-ryō, and in 839 he passed an examination for governmental service and was promoted a full three steps at once, from ju roku-i no ge (従六位下) to shō roku-i no jō (正六位上).
He held administrative positions in the Daigaku-ryō and as chief draftsman and editor (大内記) in the Ministry of the Center, and in 844 was again promoted to ju go-i no ge (従五位下). In 845, he became a teacher in the Daigaku-ryō. While in this role, he also served as vice-governor of Echigo Province and Sanuki Province, and as a tutor to Crown Prince Michiyasu, the future Emperor Montoku.
In 850, Emperor Montoku took the throne, and Koreyoshi was promoted to shō go-i no ge (正五位下). Continuing his work as a teacher, he also worked as the head of the Daigaku-ryō, director of the Capital (左京大夫), and governor of Kaga, Mimasaka, Ise, and Bizen provinces. In 855, he was awarded the rank of ju shi-i no ge (従四位下).
In the court of Emperor Seiwa, Koreyoshi continued his role as a teacher and now governor of Harima Province, and was promoted to ju shi-i no jō (従四位上). Passing through a number of other positions, including as director of the Gyōbu-shō, he was in 870 appointed as vice-minister of the Shikibu-shō. In 872, he joined the ranks of the kugyō with a promotion to sangi. In 873 he was promoted to shō shi-i no ge (正四位下) and in 879 to ju san-mi (従三位).
In 880, Koreyoshi died at the age of 69.
As a believer in Buddhist teachings, he treated people with love, exhibited extreme filial piety, and disliked killing. On his deathbed, he requested only that a Buddhist memorial service be held for him in the early winter when the ume blossomed, and otherwise spoke not a word.