All his ministerial work was carried out in North Wales. He moved to Garth Chapel, Porthmadog at the beginning of 1945. From there he went, in 1957, to Capel Tegid, Y Bala, and then, in 1962, to Moriah Chapel in Caernarfon where he remained until he retired in 1975. He wrote the history of Moriah's third half-century, Muriau Cof (1977), finishing it, as he notes in the Foreword, ‘the evening before the fire that destroyed the Chapel’ in July 1976. But this North Wales minister was well-known as an excellent preacher throughout Wales. His work as a minister was characterised by meticulous preparation: he was conscientiously studious, and an elegant composer. His preaching, too, was conscientious and elegant, but full of fervour as well. Straight-backed, clean-cut and handsome, he had a commanding presence in the pulpit and a sonorous voice with a wonderful register, what old-stagers called ‘the ancient talent of Anglesey’, and a touch of the rolling -r. He also possessed poetic and narrative rhetorical skills, polished eloquence, a gift for figures of speech, a talent for story-telling, and a stock of memorable phrases used by the preachers of yesteryear. For a man who considered preaching to be the most important work in the world, the religious relapse in the second half of the twentieth century Wales was agonising, as it was, of course, for his colleagues. From the nineteen-seventies onwards, his greatest agony was a daughter's tribulations; but it could be claimed that that enhanced his preaching powers. More than five hundred sermon-notes, written on cards, are deposited in the National Library. He also delivered a host of lectures (they too are extant), including the John Williams Brynsiencyn Memorial Lecture on “Preaching” and the Davies Lecture (on “The Devotion of Silence”: he was trying to understand the appeal of Quakerism to one of his sons-in-law). He claimed that he couldn't really lecture because every lecture became a sermon. He never held an office in the Connexion, partly because he didn't wish to do so.

He came to some prominence as a minor poet when he won the Chair at the Dyffryn Ogwen Eisteddfod for a series of lyric poems adjudicated by R. Williams Parry, a series of poems published, along with elegies composed on the death of Williams Parry and other poems, in the only volume of verse he published during his lifetime, Cloch y Bwi (Gwasg Gee, [1958]). The poems are simple well-crafted poems about everyday things, poems that no-one would claim are heartrending or terribly original. Amongst them there are charming hen benillion, a few englynion, three sonnets and a hymn (‘Pan fwyf yn teimlo'n unig lawer awr’), which became the forerunner of a small but valuable number of hymns that place John Roberts in the forefront of late twentieth-century Welsh hymn-writers (not that there are many). Like a host of preachers who took sustenance from the great eighteenth- and nineteenth-century hymn-writers, he'd regularly quote line after line from them, blending them into his sermons and his public prayers. He studied them, too. In the 1950 National Eisteddfod he won first prize for a Selection of Hymns suitable for Secondary Schoolchildren. A few years later he could have put together a volume of his own lectures on the classic hymn-writers. Although he didn't rate contemporary hymn-writing highly, soon, in the wake of his studies, using his lyrical talent and his fervent imagination, he began to compose more hymns himself, meditative hymns containing exhortation, entreaty and praise, which were, in his mind, necessary characteristics. He also said that hymns ‘needed to contain theology, … theology that had been through the furnace of experience.’ Their experiental, trustful sympathy with the human condition is what made his hymns such useful favourites with the fragile congregations of these late times. His most famous hymn is probably “Gweddi Heddiw” (‘O, tyred i'n gwaredu, Iesu da’) for which Haydn Phillips composed the prize-winning tune “Bro Aber” at the 1983 National Eisteddfod in Anglesey. A good selection of his poems and hymns were published in Glas y nef (Gwasg Gee, 1987).

He always said that he wished to be remembered as a preacher, but because of the permanence of the printed word it's more likely that he'll be remembered for his hymns. He died on 22 Nov., 1984 at Stanley Hospital, Holyhead, and was buried in the churchyard in Llanfwrog.