He was the author of an important historical work, which has seemingly perished except for some passages quoted by Bar-Hebraeus and an extract found by Assemani in Cod. Vat. 144 and published by him in the Bibliotheca orientalis (ii. 7277). He spent his earlier years as a monk at the convent of I~en-neshre on the upper Euphrates; and when this monastery was destroyed by fire in 815, he migrated northwards to that of Kaisfim in the district of Samosata. At the death of the Jacobite patriarch Cyriacus in 817, the church was agitated by a dispute about the use of the phrase heavenly bread in connexion with the Eucharist. An anti-patriarch had been appointed in the person of Abraham of Kartamin, who insisted on the use of the phrase in opposition to the recognized authorities of the church.
The council of bishops who met at Raqqa in the summer of 818 to choose a successor to Cyriacus had great difficulty in finding a worthy occupant of the patriarchal chair, but finally agreed on the election of Dionysius, hitherto known only as an honest monk who devoted himself to historical studies. Sorely against his will he was brought to Raqqa, ordained deacon and priest on two successive days, and raised to the supreme ecclesiastical dignity on August 1. From this time he showed the utmost zeal in fulfilling the duties of his office, and undertook many journeys both within and without his province. The ecclesiastical schism continued unhealed during the thirty years of his patriarchate. The details of this contest, of his relations with the caliph Mamtin, and of his many travels including a journey to Egypt, on which he viewed with admiration the great Egyptian monuments,are to be found in the Ecclesiastical Chronicle of Bar-Hebraeus. He died in 848, his last days having been especially embittered by Mahommedan oppression. We learn from Michael the Syrian that his Annals consisted of two parts each divided into eight chapters, and covered a period of 260 years, viz, from the accession of the emperor Maurice (582-583) to the death of Theophilus (842-843).
In addition to the lost Annals, Dionysius was from the time of Assemani until 1896 credited with the authorship of another important historical work a Chronicle, which in four parts narrates the history of the world from the creation to the year AD 774-775 and is preserved entire in Cod. Vat. 162. The first part (edited by Tullberg, Upsala, 1850) reaches to the epoch of Constantine the Great, and is in the main an epitome of the Eusebian Chronicle. The second part reaches to Theodosius II and follows closely the Ecclesiastical History of Socrates; while the third, extending to Justin II, reproduces the second part of the History of John of Asia or Ephesus, and also contains the well-known chronicle attributed to Joshua the Stylite. The fourth part is not like the others a compilation, but the original work of the author, and reaches to the year 774-775 apparently the date when he was writing.
On the publication of this fourth part by M Chabot, it was discovered and clearly proved by Noldeke (Vienna Oriental Journal X. 160-170), and Nau (Bulletin critique, xvii. 321-327), who independently reached the same conclusion, that Assemani's opinion was a mistake, and that the chronicle in question was the work not of Dionysius of Tell-Mahre but of an earlier writer, a monk of the convent of Zulfnin near Amid (Diarbekr) on the upper Tigris. Though the author was a man of limited intelligence and destitute of historical skill, yet the last part of his work at least has considerable value as a contemporary account of events during the middle period of the 8th century.