The short answer is: we simply don't know with any certainty, at least prior to the Late Middle Ages (which I'm not qualified to speak to).

Longer answer: depends on what you mean by mercenary.

If you mean large ethnic/tribal/regional groups under native leaders, possibly with non-combatants in tow, that was a thing going back into the earliest part of the Middle Ages. The late Romans made extensive use of foederati - allied peoples, usually subsidized in land or cash. Anglo-Saxons were (probably) initially brought over to Britain in a similar arrangement. Jumping forward in time, considerable numbers of Normans were hired to fight in Italy in the 11th century. In the former two cases, negotiation between the contracting leadership and the contracted leadership would be usual. In the latter, it seems that bands of mercenaries - though, perhaps, adventurers would be a better term, as they lacked the stigma of the later mercenary companies - ventured south on their own dime and found employment once there.

Similarly, if by mercenaries, we mean individuals fighting primarily for pay, that too was probably always a thing. Certainly, as far back as before the Norman conquest the dukes of Normandy were hiring men to garrison castles.

The third option - small to middling bands, almost always commanded by non-aristocrats and without clear political affiliation - is the most commonly referred to, and also one of the trickiest to discuss. We know they were used in great numbers, especially in the 12th century. Stephen and Matilda used them in the Anarchy; Henry II, Richard I, and Philip II used them in their continental wars. At times, such groups probably formed the backbones of Anglo-Norman armies, especially in the sort of prolonged low-intensity fighting that characterized Richard's early reign as Duke of Aquitaine.

What we can say about them in the context of your question is fairly limited. They had leaders who rulers contracted with. They could be available fairly quickly, and given the state of travel at the time, probably were relatively nearby. They were often hired for a campaign, on a temporary basis, but Richard I employed one such mercenary leader, Mercadier, for his entire adult life.

I hate to leave you with such an incomplete answer, but the simple truth is, we don't know much about these men. They don't leave us any records of themselves. Almost the only time they come up is in the context of aristocrats, and then they're usually painted in an extremely negative light. Even details of origin are made unknowable due to the medieval convention of stereotyping such men - thus all mercenaries in a given region and period become Brabanters, or Gascons, or whate have you. So we know when a king or a duke hired mercenaries; we know the nasty exploits the mercenaries are supposed to have gotten up to; sometimes, we even learn about the violent ends they came to. But precious little more than that do we really know.