Rhodri was born in the early 1230s. His father was Gruffydd ap Llywelyn, the illegitimate but eldest son of Llywelyn the Great, and his mother was Senena ferch Caradog. Little is known about his early life except that on his grandfather's death in 1240 his uncle Dafydd ap Llywelyn (his father's younger, but legitimate, half brother) inherited the throne and immediately imprisoned his father, thus disinheriting him. Gruffudd was subsequently relinquished by Dafydd under the terms of the Treaty of Gwerneigron following Henry III's invasion of 1241, and taken to London as a hostage where he died in 1244 during a failed attempt to escape from the Tower of London. Also in 1241, Rhodri along with his brother Dafydd ap Gruffydd were sent as hostages to King Henry. Dafydd ap Llywelyn seized the opportunity to wage war against Henry, defeating him in battle in the summer of 1245 during Henry's second invasion of Wales.