If you're asking if there were Saxons/Saxon kingdoms before Mercia the answer is yes.

According to the Anglo-Saxon Chronicle, a book of the history of the Anglo-Saxon people compiled by King Alfred in the 9th century, the brothers Hengist and Horsa were the first to arrive in England.

This took place in the mid 5th century. Hengist and Horsa, two brothers who are thought to have been of Jutish decent were hired by the British King Vortigern to fight against the Picts and Scots who were invading.

With the retreat of the Roman military from England the native Romano-Britons were pretty defenseless. The most natural response is to hire mercenaries, and what better mercenaries than Germans.

the Chronicle tells is that the brothers and their warband were granted the Isle of Thanet in east Kent, and that the Saxons eventually began demanding more monthly allotments from the Britons but were refused.

The outbreak of war between the former mercenaries and the Britons was devastating. Horsa himself was killed in battle, and Hengist massacred the British nobility at a peace conference (Night of the Long Knives).

The Saxons won the war and carved out the Kingdom of Kent circa 488, almost 40 years before the Kingdom of Mercia.

Much of the information in the chronicle is pulled from Bede's Ecclesiastical History of England, published a century earlier. The earliest mention of these events was a British monk, Gildas the Wise, in the mid 6th century.

Since both Britons and Saxons seem to agree over a period of 400 years, despite their obvious biased towards each other, it can be concluded that these events are pretty factual, at least in the broader pictures they convey.

TL;DR The Britons asked the Saxons to fight, the Saxons wanted paid more, they rebelled and treacherously murdered the British nobility at a peace conference, winning the war and heralding the beginning of Anglo-Saxon dominance of Britain