After Prince Tsunesada was removed from the position of Crown Prince in the Showa Incident of 842, instead the Emperor Montoku succeeded to the throne after Emperor Nimmyo died (850), with the recommendation of his uncle, FUJIWARA no Yoshifusa, who had contributed to the resolution of the incident.
承和9年（842年）、承和の変で皇太子恒貞親王が廃されると、変の解決に功のあった伯父藤原良房にも推され代わりに立太子し、嘉祥3年（850年）、仁明天皇の崩御に伴い即位。