Author of a number of historical works, one of which (the Ymagines historiarum) contains important information about Outremer, especially from 1185 onward.
Although not a major source on the Third Crusade (1189-1192), Ralph’s work contains valuable comparative information, of a high level of reliability. His surname, Diceto, probably refers to Diss in Norfolk, although this is not certain. After holding a number of ecclesiastical positions and studying at Paris, in 1180 he was elected dean of St. Paul’s Cathedral, London. His important position assisted him in collecting information for his historical writings: for example, in 1196 William Longchamp, chancellor of King Richard I of England, sent him a copy of the supposed letter of the Assassin leader known as the Old Man of the Mountains to Leopold V, duke of Austria, giving an explanation of the assassination of Marquis Conrad of Montferrat in 1192. Ralph inserted this letter into his Ymagines. His own chaplain gave him details about the foundation of the English military order of St. Thomas of Acre. Some of the letters he recorded survive nowhere else. Although his information on Outremer was secondhand, his account was generally accurate because he either reproduced letters from the East in full or summarized them.