Sparks was ordained in the Church of England as a deacon on 24 September 1933 by Thomas Strong, the Bishop of Oxford, and as a priest in 1934. From 1933 to 1936, he combined his curacy at All Saints Church, Oxford, and a position as an honorary chaplain of Ripon Hall. Additionally, he assisted H. J. White with the production of a new version of the VulgateNew Testament between 1933 and White's death in July 1934. He resigned from Ripon Hall in 1936 because of the increasing influence of the Modern Churchmen's Union on the theological college. In 1936, Sparks left Oxford and moved to Durham in the north of England. He was a lecturer in the theology at Durham University between 1936 and 1946. He specialised in teaching Christian doctrine and Patristics, and later also taught Hebrew. During World War II, he was additionally censor of Hatfield College and University College: the two colleges had been combined for the duration of the war. At the end of the war, Sparks was looking to move again. He was interviewed for the Samuel Davidson Professorship of Old Testament Studies at the University of London, but was not successful. He then applied for the appointment of Cadbury Professor of Theology at the University of Birmingham. He was successful and took up the chair in October 1946. He developed the theology syllabus and expanded the department's academics to create an ecumenical faculty; this included the first Roman Catholic, H. Francis Davis as a visiting lecturer. From 1947 to 1952, he was Dean of the Faculty of Arts. In 1949, he was awarded a Doctor of Divinity degree by his alma mater, the University of Oxford: the DD is the most senior degree awarded by the University. In November 1951, Sparks was elected Oriel Professor of the Interpretation of Holy Scripture at the University of Oxford. He took up the appointment on 1 October 1952. The chair had been linked with a canonry at Rochester Cathedral but this was separated before the 1951 election. The chair remained linked with Oriel College, Oxford and he was duly elected a fellow of the college. He gave a number of lecture series through the Faculty of Theology and Religion. In addition, though he was not required to, he provided tutorials to undergraduate theology students at his college. From 1953 to 1977, he was Editor of the Journal of Theological Studies. In 1957, G. N. Clarke retired as Provost of Oriel College. Sparks was encouraged to apply to replace him, but upon learning that it would mean giving up his chair, chose not to. In 1962, Sparks was the president of the Society for Old Testament Study. From 1961 to 1968, he was Rector of All Saints Church, Wytham, in addition to his academic positions. In 1976, he retired from academia.
Sparks lived with his mother until her death in 1951. On 25 August 1953, he married Margaret Joan Davy. Having been born in 1930, she was more than two decades younger than her husband. Together they had three children: two sons and one daughter.