The son of William Laing, a bookseller in Edinburgh, where he was born, he was educated at Canongate Grammar School and then attended the University of Edinburgh. At fourteen he was apprenticed to his father.
Shortly after the death of the latter in 1837, Laing was elected to the librarianship of the Signet Library, a post he retained till his death. Apart from general bibliographical knowledge, Laing was best known as a student of the literary and artistic history of Scotland. Laing was struck with paralysis in 1878 while in the Signet Library, and it is said that, on recovering consciousness, he looked about and asked if a proof of Wyntoun had been sent from the printers.
He died a few days afterwards, in his eighty-sixth year. His library was sold at auction by Sotheby, Wilkinson & Hodge over a period of thirty-one days, and realized £16,137. He bequeathed his collection of manuscripts to the University of Edinburgh.