Carew had a considerable reputation as an antiquary and was a friend of William Camden, John Cotton, and Thomas Bodley. He gathered a large collection of materials relating to Irish history and pedigrees, which he left to his secretary, Sir Thomas Stafford (supposed to be his illegitimate son). A portion has disappeared, but 39 volumes that came into Laud's possession are now held in Lambeth Palace Library, and a further four at the Bodleian Library. A calendar of the former was published in six volumes between 1867 and 1873, edited by J. S. Brewer and W. Bullen, under the title "Calendar of the Carew Manuscripts preserved in the Archiepiscopal Library at Lambeth". Carew's correspondence from Munster with Sir Robert Cecil was edited in 1864 by Sir John Maclean for the Camden Society, and his letters to Sir Thomas Roe (1615–1617) in 1860.