Joscelyn was involved in Parker's attempts to secure and publish medieval manuscripts on church history, and was one of the first scholars of the Old English (Anglo-Saxon) language.

Joscelyn was born in 1529, and was the son of Sir Thomas Joscelin and Dorothy née Gate, of Hyde Hall, Sawbridgeworth.

In 1555, during Queen Mary I's reign, Joscelyn subscribed to the required church doctrine, and was once more a teacher of Greek during the academic year 1556–1557.

From Parker's interest in the history of early Christianity, and to discover more information about the growth of papal power in the Middle Ages, Joscelyn also began to study Old English (a topic of interest to Parker), and helped the archbishop in his studies of the English pre-Norman Conquest church.

He had a good understanding of the law codes of the English Anglo-Saxon kings, which he used in the preparation of an Old English-Latin dictionary he worked on, but which was never completed.

Although Parker claimed in a letter that he was the author, it is likely that at the very least Joscelyn did most of the research, and the manuscript of the work, which is now Vitellius E xiv, is largely in his handwriting.

He contributed extensively to Parker's A Testimonie of Antiquitie Shewing the Auncient Fayth in the Church of England, the earliest printed book containing portions in Old English.