In 1718 the feoffees offered Byrom the post of Library Keeper, an offer which he declined. Three years later he married his cousin Elizabeth Byrom, and they settled in Manchester. John, however, continued to spend long periods of time in London. He was elected a Fellow of the Royal Society in 1724 and remained in touch with some of the leading intellectuals of the day.
An enthusiastic collector of books, Byrom was also a good friend of Robert Thyer, who became Librarian of Chetham's in 1732. Indeed, Byrom frequently acted as an agent for the Library, purchasing books at London auctions.
Byrom's own library, consisting of forty manuscripts (including the manuscript of his poem Christmas Day, which became the Christmas carol, Christians Awake!) and some 2,800 printed books, was presented to Chetham's by Eleanora Atherton, a descendant, in 1870. It is essentially a working collection, with a strong emphasis on theology, although science, medicine, literature, languages and shorthand are also represented.