In 1953 Ellegård published his PhD thesis, which dealt with the development of the use of the auxiliarydo in the grammar of late Middle English and Early Modern English. This thesis has remained an influential and frequently cited work central to the study of this aspect of the historical syntax of English. He was appointed associate professor of English in the same year. He was a professor of the English language at Gothenburg University between 1962 and 1984. His book Darwin and the General Reader is a detailed study of the changes in British public opinion which took place following the 1859 publication of The Origin of Species. It is based on an analysis of the periodical press of the period. In 1977, he was elected member of the Royal Swedish Academy of Letters, History and Antiquities.
After retirement Ellegård started a new line of research into the historicity of Jesus and the idea that Jesus is a myth. In his book Myten om Jesus , he presents new theories about the Dead Sea Scrolls and their association with the early history of Christianity. Ellegård argues that the Jesus of the gospels is a mythical figure and that the gospels are largely fiction. He identifies the figure Paul of Tarsus had a vision of as corresponding to the Essene Teacher of Righteousness, the leader of the Essenes at Qumran about 150 years before the gospels, and writes that it was Paul who created Christianity through his contacts with the sect that kept the Dead Sea Scrolls. According to Ellegård, the Damascus Document gives support to this theory. The document states that the Essenes moved to Damascus outside Jerusalem, but the word "Damascus" appears to being used symbolically to refer to exile. Ellegård interprets this as evidence that the "Damascus" that is mentioned in the Acts of the Apostles in fact is Qumran. Paul was on his way to Damascus when he had a vision of Jesus.
Selected publications
The Auxiliary Do: The Establishment and Regulation of its Use in English
Darwin and the General Reader
A Statistical Method for Determining Authorship
The Syntactic Structure of English Texts
«Who were the Eruli?» Scandia 53, 1987
Myten om Jesus: den tidigaste kristendomen i nytt ljus, Bonniers.
Jesus – One Hundred Years Before Christ: A Study In Creative Mythology,.