On 29 August 2019, Forster was elected as Bishop of Derry & Raphoe, following the retirement of Bishop Kenneth Good In 31 May 2019. His consecration took place on the evening of Sunday 8 December in St Patrick's Cathedral, Armagh. He was enthroned in the two diocesan cathedrals in Derry and Raphoe at the beginning of January 2020.[citation needed]
Forster was President of the Irish Council of Churches, his term of office ran from March 2022 until March 2024. He was succeeded by Bishop Sarah Groves.
He is married to Heather and has three children, Hannah, Patrick & Megan (in age order). [citation needed]