Trần Thủ Độ was a powerful official in his own right and a cousin of Trần Thị Dung, mother of the empress Lý Chiêu Hoàng. He used his influence with the royal family first to get the seven-year-old Lý Chiêu Hoàng appointed to the throne by her ailing father Lý Huệ Tông, and then to arrange a marriage between the new empress and his nephew Trần Cảnh, also aged seven. Trần Thủ Độ' then arranged for many members of the royal family to be killed, manipulated the marriage between Trần Cảnh and Lý Chiêu Hoàng, and appointed 'Trần family members to key posts at court, all to clear the way to the throne for his nephew. Trần Cảnh ascended to power in 1225 or 1226 when Lý Chiêu Hoàng yielded the throne to him. Still only eight years old, he ruled for 33 years.
Following Trần Thái Tông's ascendancy, Trần Thủ Độ had himself appointed grand chancellor, a position from which he exerted considerable power. During his proxy rule, Trần Thủ Độ enacted major civil reforms including the division of the Vietnamese kingdom into 12 provinces and the institution of various taxes on land holdings and transactions. In 1258, he was responsible for a reaching a peaceful settlement with Mongol invaders from the north. Trần Thủ Độ also took it upon himself to secure the new Trần Dynasty by wiping out as many family members of the previous Lý Dynasty as possible, and driving others into hiding or exile.