Joscelyn was born in 1529, and was the son of Sir Thomas Joscelin and Dorothy Gate. John was their third son to survive childhood, and was probably born on his father's estate at High Roding, Essex. He attended Queen's College at Cambridge beginning in 1545, attaining a Bachelor's of Arts in 1549. In the school year 1550–1551 he taught Latin at Queen's College, and the following school year he taught Greek. At the end of 1552, he was awarded a Master of Arts. 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 school year 1556–1557. However, in 1557 he resigned from his fellowship at Queen's College.