It appears that Cynegils became king on the death of King Ceolwulf c. 611. His relationship to Ceolwulf is uncertain. Cynegils is variously described in West Saxon sources as being a son of Ceolwulf, a son of Ceol brother of Ceolwulf, a son of Ceola son of Cutha, a son of Cuthwine son of Ceawlin, and a son of Cuthwulf son of Cuthwine. Several of the sources give Cynegils a brother named Ceolwald, described as the grandfather of King Ine. Although the Anglo-Saxon Chronicle and Wessex king lists portray the West Saxons as ruled by a single king, it is likely that the kingship was shared between two or more kings.
The Anglo-Saxon Chronicle for the year 611 states: "This year Cynegils succeeded to the government in Wessex, and held it one and thirty winters. Cynegils was the son of Ceol, Ceol of Cutha, Cutha of Cynric." Contradicting this simple account, the entry under 614 states that "This year Cynegils and Cwichelm fought at Beandun, and slew two thousand and forty-six of the Welsh." Likewise, Bede writes that the attempted assassination of King Edwin of Northumbria in 626 was ordered by Cwichelm, king of the West Saxons. Whether the King Cwichelm of 614 is the same person as the king of the late 620s, and whether this person is the same as the Cwichelm who was baptised, and died, c. 636, is disputed. Some historians presume Cwichelm to have been a son of Cynegils.
In 628, Cynegils and Cwichelm fought King Penda at Cirencester. The Chronicle could be expected to report a victory, but does not, so it is likely that Penda was the victor. Cynegils and Cwichelm appear to have been subject to Edwin of Northumbria by this time, paying an enormous tribute of a hundred thousand hides if Nick Higham's conception of the Tribal Hidage's origins is correct.