Well, in a sense you could say Cortes was acting as a mercenary on behalf of anti-Aztec tribes, before he went all out and took over the empire. Through his alienness, he stood out enough to become a leader, but his success was completely predicated on the inter-tribal tensions that had existed for generations before his arrival.

The Aztec Empire had recently subsumed the neighboring tribes and forced them to pay a tribute of humans for the Aztecs to sacrifice to their gods. You can learn more about this in How to piss off your subject peoples 101. When Cortes showed up, he learned about this in Tlaxcala-once a powerful independent state, but at the time subjugated by the Aztec. Essentially, he took advantage of his technology, alienness, etc to foment anti-Aztec sentiment in their empire and was able to lead an army of Mexican natives against their imperial overlords.

I know this probably isn't exactly what you're looking for, but the Cortesian overthrow of the Aztecs began, from the perspectives of the natives, as a war of independence, using Cortes as a catalyst for their cause. It was only after disease and Spanish reinforcements that Spain herself was truly able to conquer the Americas for herself.

Follow up question: what about in Africa or Asia? Both areas have seen periods of political turmoil, were any groups (specifically Europeans) of mercenaries used to sway local conflict to one side of another?