He witnessed the events leading up to Richard III being offered the English crown.
He called it: De Occupatione Regni Anglie per Riccardum Tercium ('The Occupation of the Throne of England by Richard III').
[1] The account is a major source of information about the period, but it remained in a French library in Lille until rediscovered in 1934 and published by C. A. J.
[2] Mancini's report was written for Angelo Cato, Archbishop of Vienne, one of the counsellors of King Louis XI of France[2] and also his doctor and astrologer.
Mancini's report was lost for centuries but was discovered in the Municipal Library in Lille, France, in 1934.