Walker was the drummer of the Grenadier Coy when it mustered on 27 October 1767 at Philadelphia. He embarked with the regiment from Cork on 1 May 1767 and arrived at Philadelphia in July 1767. He traveled with the regiment to Ft. Pitt in the spring of 1768 and went down the Ohio to the Illinois Country. He was listed as on command in the April 1768 muster, so he may have been among the advance party that went to Ft. Pitt prior to the main body of the regiment. Once in Illinois, Walker was among those who succumbed to the sickness rampant in the Illinois Country. He died at Ft. Chartres on 10 January 1769. He was listed as a private under the casuals on the April 1769 muster as private, but he almost certainly remained a drummer until his death. His replacement was appointed on 5 February 1769.
Even a man with a simple last name like Walker could have his name appear in multiple ways on muster rolls. It isn't clear if different men were filling out the returns or if the clerk simply wasn't being careful or potentially the orderly sergeant was dictating the return from another list or roster and the clerk misheard the name.