Egfrith was much younger than Etheldreda and the marriage was not happy. After twelve years she obtained her husband's consent to retire from worldly affairs and she entered the convent of the Abbess Ebba, King Egfrith's aunt, at Coldingham, where she received the veil and clothing of a nun at the hands of Bishop Wilfrid. A year later she set out for the Isle of Ely.
At that time there was no one living where the city of Ely now stands, but there was a village named Cratendune.Etheldreda chose a site about a mile to the north of the village There she founded a monastery for both nuns and monks in the year 673. The inhabitants of Cratendune abandoned their village and moved to the vicinity of the monastery. Thus it is possible to say exactly when Ely began to exist.
The project was assisted by Bishop Wilfrid, who consecrated Etheldreda abbess of the new monastery, which she continued to rule for some years, setting an example by her piety and disdain for comfort. Hot baths were regarded by her as a luxury and she took only about four a year, and those at times of important religious festivals.
She became a victim of plague which caused a large swelling in her throat. This was lanced by her surgeon, but she died on 23rd June, 679. Sexburga, succeeded her as abbess.