I've heard of them employing Chinese mercenaries before, but, given their expansion into Eastern Europe, would they have ever recruited mercenaries from Europe? The Rus', Baltic, and Balkan regions seem to be likely candidates to me personally, though I have yet to find anything confirming this idea.
It is true that the Mongol army consisted not solely of the Mongols, but also of several ethnic, especially of nomadic groups across Eurasia. In his latest introductory book on the Mongol Empire, May, authority of the Mongol Empire, cites the description of Marco Polo as a testimony of this not so homogeneous nature of the Mongol invasion army: '......they [the Mongols] conquered a good 8 provinces but did them no harm, nor did he [Chinggis Khan] strip them of their things. But he took them with him to conquer other people. In this way, he conquered this great multitude of peoples, as you have heard' (May 2019: 43).
I'm not so sure, however, whether this kind of relationship between the Mongols and their newly subjugated group (or 'ally') can be classified under the category of 'mercenaries'.
As for Western Eurasia, as I wrote a brief notice in this thread, it is relatively well known that the Mongols incorporated some steppe people in SE Russia originally known as the Polovstsians or the Cumans into the ruling group in Russia (the Jochi Ulus), the Golden Horde. The another name of the Golden Horde, 'the Kipchak Khanate' originally came from the byname of these groups.
On the other hand, some dependent (by subjugation) polity of the Mongols and their rulers made use of the Mongols' new formidable military power in order to expand their political influence in their region: As I noted in this tree thread of daily questions before, King Hethum I of Armenian Cilicia can be regarded as one of such rulers those who took a dangerous bet.
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