On 22 September 1737, Ducarel was elected a fellow of the
Society of Antiquaries of London, and he was one of the first fellows of the society nominated by the president and council on its incorporation in 1755. He was also elected a member of the Society of Antiquaries at
Cortona on 29 August 1760, was admitted a fellow of the
Royal Society of London on 18 February 1762, became an honorary fellow of the Society of Antiquaries of
Cassel in November 1778, and of the
Society of Antiquaries of Scotland in 1781. In 1755, he failed to obtain the post of sub-librarian at the
British Museum; but in 1757 he was appointed keeper of
Lambeth Palace Library by
Archbishop Hutton. His predecessors in this post (who had included
Henry Wharton,
Edmund Gibson and
David Wilkins) had all been clergymen who treated the post as a part-time responsibility and as a stepping-stone to more lucrative ecclesiastical preferments. Ducarel, by contrast, remained in post for nearly thirty years, under five archbishops (
Herring,
Hutton,
Secker,
Cornwallis, and
Moore), until his death. He greatly improved the catalogues both of the printed books and the manuscripts at Lambeth, and made a digest, with a general index, of all the registers and records of the
province of Canterbury. He was assisted by his friend,
Edward Rowe Mores, the Rev. Henry Hall (his predecessor in the office of librarian), and the engraver
Benjamin Thomas Pouncy, who was for many years his clerk and deputy librarian. Ducarel's contribution was seriously impeded by his complete blindness in one eye, and the weakness of the other. Besides the digest preserved among the official archives at Lambeth, he formed another personal manuscript collection in forty-eight volumes: after his death this passed to the antiquary
Richard Gough, and in 1810 was bought for the
British Museum library. He also took a more general interest in the ecclesiastical antiquities of the
province of Canterbury, and, with Mores, compiled a history of
Croydon Palace and of the town of
Croydon. This was completed and presented to Archbishop Herring in manuscript in 1755, and published in 1783. However, the work led to a virulent rift between the two friends, when Mores, who had made significant contributions to it, discovered that he was not named on the title page. In 1763, Ducarel was appointed by the government, with Sir
Joseph Ayloffe and
Thomas Astle, to sort and catalogue the records of the
state paper office at
Whitehall, and afterwards those in the
augmentation office. On the death of
Archbishop Secker in 1768 Ducarel applied for the post of secretary to the new archbishop,
Frederick Cornwallis, but without success. ==Wider antiquarianism==