Thingfrith was the father of King Offa. He was the son of Eanwulf and descended from King Penda of Mercia's brother, Eowa of Mercia. A tradition related by the 13th century Vitae duorum Offarum names king Offa's mother as Marcellina, who would thus be wife of Thingfrith were the tradition authentic.
Both the Anglo-Saxon Chronicle and the Anglian collection give the ancestry of Offa, naming Thingfrith. The version in the Chronicle reads, "That Offa was Thingfrith's offspring, Thingfrith Eanwulf's offspring, Eanwulf Osmod's offspring, Osmod Eawa's offspring, Eawa Pybba's offspring, Pybba Creoda's offspring, Creoda Cynewald's offspring, Cynewald Cnebba's offspring, Cnebba Icel's offspring, Icel Eomer's offspring, Eomer Angeltheow's offspring, Angeltheow Offa's offspring, Offa Wermund's offspring, Wermund Wihtlaeg's offspring, Wihtlaeg Woden's offspring".
- Mrs. Mathew Hall (1854). Lives of the Queens of England Before the Norman Conquest. Blanchard and Lea. pp. 297–. Retrieved 29 May 2013.
- Erik Kooper (2002). The Medieval Chronicle II: Proceedings of the 2nd International Conference on the Medieval Chronicle, Driebergen/Utrecht 16-21 July 1999. Rodopi. pp. 172–. ISBN 978-90-420-0834-2. Retrieved 29 May 2013.