Later, Mapenor served as Dean of Hereford before being elected as bishop against the wishes of King John of England.

During his short episcopate, he supported John's son and successor King Henry III of England, and was active in his diocese, as a number of surviving documents show.

[3] During his time at Hereford, he was the subject of the Prose Salernitan Questions, which compared his sexual powers to three other clerks.

His election had been overseen by the papal legate Guala Bicchieri,[5] who was also assigned the case by the papacy after John objected.

The bishop was also present when the king and Llywelyn of Gwynedd, a Welsh prince, concluded a peace treaty at Worcester in March 1218.

The king employed Mapenor in April as a diplomatic escort, ordering the bishop to bring some of the southern Welsh princes to Woodstock to swear fealty to Henry III.

One of his surviving charters documents a licence given to Leominster Priory as alms for the souls of his parents and his predecessor as bishop, Giles de Braose.