Mercia was an early medieval kingdom in what is now the Midland region of England. In the mid-seventh century under King Penda and again in the eighth under King Offa, it was among the most powerful kingdoms in Britain. Cearl is the first king of Mercia recorded in the surviving sources. What can we say about him?
Among these were Osfrid and Eadfrid, sons of King Edwin, who were both born to him in exile of Coenburg, daughter of Cearl, King of the Mercians
--Bede, Ecclesiastical History, Book II Ch. 14
The line quoted above is the sole mention of Cearl in any historical source, which is not a lot to go on, even by the standards of early medieval history. However, a few more things can be inferred.
Cearl had a daughter, Coenburh, who had borne two sons before 617 AD. Even if Coenburh was married young, say at age 15, and conceived her first child straight away, she can hardly have been born much after 600 AD and a few years before is more likely. By extension, her father Cearl cannot have been born much after 580, and some years before is more likely.
Or, possibly, lack thereof. Cearl is not mentioned in the genealogy of the Mercian kings in the Anglian Collection or Historia Brittonum Ch 60. Either he was not in the direct line of ancestry of the later kings, perhaps belonging to a different branch of the royal family, or he belonged to a different dynasty altogether. There is a group of three C- names in the Anglian Collection genealogy (Creoda, Cynewald, Cnebba), placed in the generations immediately before Penda's father Pybba. Alliteration was popular, though by no means universal, in Old English naming conventions, so these may indicate that Cearl belonged to a branch of the dynasty that claimed descent from one or other of these figures. But this is essentially speculation.
Cearl married his daughter to a political exile from Deira who was on the run from Aethelferth of Bernicia/Northumbria at the time. Aethelferth's military power is well documented. Bede describes him as very powerful and ambitious and credits him with military success over much Brittonic territory (Ecclesiastical History Book I Ch. 34). Aethelferth had won a major victory over the Irish of Dal Riada (roughly modern Argyll) in 603, and would win another over King Selyf of Powys at the Battle of Chester in 613/617 (see article on the Battle of Chester for discussion of the date), both kingdoms well removed from his core territory of Bernicia (approximate location map for Bernicia, Dal Riada and Powys here). Evidently Aethelferth was well capable of projecting his military power over considerable distances, with disastrous consequences for those on the receiving end. Yet Cearl of Mercia chose to form a marriage alliance with Aethelferth's sworn enemy. Assuming he did it knowingly and was not stupid, this implies the following things to me:
If correct, this further implies that Cearl was ambitious, that he fancied expanding his power to the north (at least into Eadwine's ancestral territory of Deira, and perhaps beyond that into Aethelferth's own kingdom), and that he thought he had the means to do so.
Cearl's reign was certainly over by 633 AD, when Penda son of Pybba was ruling Mercia (Bede Book II Ch. 20). The date and circumstances of the end of Cearl's reign are not known.
It is known that Cearl's son-in-law Eadwine had changed his place of exile to the court of Raedwald of East Anglia before 617 (Bede, Book II Ch. 12). This may indicate that Cearl had simply changed his mind, perhaps having been threatened by Aethelferth (as we know Raedwald was) and decided that discretion was the better part of valour, or perhaps having decided that Eadwine was no longer a good bet or having taken a dislike to him.
Or it may indicate that Cearl had died or been deposed before 617, and his successor no longer wanted Eadwine in Mercia. The next king of the Mercians mentioned by Bede is Penda son of Pybba, who was ruling in Mercia by 633 AD. If the change of initial letter in the names of the Mercian kings from C- to P- is indicative of a change of dynasty, and if Eadwine's departure indicates a change of political policy (very likely, since it is clear from Bede's account that Penda and Eadwine were enemies), it is possible that Cearl was deposed as King of the Mercians by Penda or Penda's predecessor at some time before 617.
It is not clear whether Penda was Cearl's direct successor, or if there were one or more kings in between. Bede describes Penda in 633 as "a warrior of the Mercian royal house" and says that "from this time on" he ruled the Mercians, which would suggest that Penda became king in or around 633. If this is the case, it suggests that if Cearl was deposed before 617, it was by Penda's predecessor(s) rather than by Penda himself.
Even if Cearl was deposed by a rival, his family may not have vanished entirely from Mercian history. A later king of the Mercians, Coenred, who reigned in 704-709 (Bede Book V Ch. 24), had the same first name element as Cearl's daughter Coenburh. This may be pure coincidence, or it may indicate a connection with Cearl's family.
Bede, Ecclesiastical history of the English people. Translated by Leo Sherley-Price. Penguin Classics, 1968, ISBN 0-14-044565-X.