Diarmaid was born in Kent, the only child of Rev. Nigel J.H. MacCulloch T.D. and Mrs. Jennie MacCulloch (née Chappell). his father was an army chaplain until 1956 when he took the country living of Wetherden (Suffolk), where Diarmaid was brought up and lived until 1972, on his father's retirement as Rector of Wetherden and Vicar of Haughley.
Diarmaid spent 1969-78 at Churchill College, Cambridge (with 1972-3 at the University of Liverpool receiving a Diploma in Archive Administration); for his doctorate Diarmaid studied under the great Tudor historian Sir Geoffrey Elton.
Diarmaid joined the Gay Christian Movement (as it was then called) in 1976, soon after its foundation, and later served two terms on its committee and also briefly as Honorary Secretary. In 1978 Diarmaid moved to teach at Wesley College, Bristol.
He taught Methodist ordinands there; with a year's interval in 1986-7 at Ripon College Cuddesdon prior to ordination in the Church of England as Deacon and subsequently stopped in 1988 from further work in the Church by the atmosphere of hysteria surrounding the Tony Higton-inspired Synod motion.
He resigned from Wesley in 1990 in order to devote himself to freelance teaching, research and writing on Reformation History. He accepted Oxford University's offer of a lectureship in 1995 and has been there ever since, as a Fellow of St. Cross College and since 1997 also Professor of the History of the Church. Diarmaid has been Senior Tutor of St. Cross College and holds the degree of Doctor of Divinity from Oxford.
Diarmaid is a Fellow of the Society of Antiquaries of London (1978), Fellow of the Royal Historical Society (1982) and Fellow of the British Academy (2001). His hobbies are viewing historic monuments, singing, keyboard playing and drinking beer.