Human skulls (and death’s heads: skulls lacking the lower jaw) are very rare charges in period heraldry, but they do show up occasionally. The following is a rundown of all the examples I’ve found so far.
The earliest examples are the attributed arms of Awil-Marduk, son of Nebuchadnezzar II, which started appearing in armorials in the mid-15th C (usually recorded as Ewfullmer or something similar).
Starting in the late 15th C, there are also occasional examples of a skull (or death’s head) used in the attributed arms of death.
As for arms that may have been borne by actual people, the earliest example I’ve found so far is a death’s head in the arms of Francesco Carosio, Bishop of Melfi, as they appear in the Chronicles of the Council of Constance. The earliest version of the Chronicles I’ve found the arms in date to 1483, almost 70 years after the Council, so it’s possible these aren’t an accurate representation of the Bishop’s arms.
In the 16th C we find a couple more examples. In France we have the arms of the Count of Macoy with a skull holding a bone in its mouth; and in Germany the arms of Coeis(?), a death’s-head with a lily-of-the-valley plant.
And finally, edging into the early 17th century, there’s a German example in a crest from 1603, and the Italian arms of Bonifaci from c. 1605.