In on of the online dictionaries, it refers one to "Muleteer" when looking up "mule skinner." It lists 1870 for first usage of mule skinner. The definition for muleteer is as follows:

Main Entry: mu.le.teer
Pronunciation: "myü-l&-'tir
Function: noun
Etymology: French muletier, from mulet, from Old French, diminutive of mul mule Date: 1538
one who drives mules