The diocese is ancient, and the area of Ely was part of the patrimony of Saint Etheldreda. A religious house was founded in the city in 673. After her death in 679 she was buried outside the church, and her remains were later reburied inside, the foundress being commemorated as a great Anglian saint.
The diocese has had its boundaries altered various times. From an original diocese covering the historic county of Cambridgeshire and the Isle of Ely, Bedfordshire and Huntingdonshire were added in 1837 from the Diocese of Lincoln, as was the Sudbury archdeaconry in Suffolk from the Diocese of Norwich. In 1914 Bedfordshire became part of the Diocese of St Albans, and western Suffolk became part of the Diocese of St Edmundsbury and Ipswich, whilst Ely took a western part of the Diocese of Norwich. Peterborough remains the seat of the Diocese of Peterborough.
Today the Diocese covers an area of 1,507 square miles (3,900 km2). It has a population of 705,000 and comprises 209 benefices, 303 parishes and 335 churches with 145 stipendiary parochial clergy.
There are also six retired bishops living in the diocese who are licensed as honorary assistant bishops: