He may very well have served in the Mexican War, but may have been exempted from CW Conscription by the 20 slave exemption law.

Accordingly, at 6 o'clock on the morning of the 26th, we crossed Leaf River, burning the bridge behind us to prevent any enemy who might be in pursuit from following; thence through Raleigh, capturing the sheriff of that country, with about $ 3,000 in Government funds; thence to Westville, reaching this place soon after dark. Passing on about 2 miles, we halted to feed, in the midst of a heavy rain, on the plantation of Mr. Williams.
OR V24, Pt. 1, p. 526

….

“As day faded into twilight, the horse soldiers rode through Westville, halting for their evening meal in midst of a heavy downpour on the George W. Williams plantation, two miles west of the village, near Strong River.“ [OR V 24, Pt. 1, pp. 525/26]

The Vicksburg Campaign, Vol. II, Grant Strikes A Fatal Blow, Bearss, pp. 212/13

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1860 Simpson County Slave Schedule - Mississippi
NAME OF SLAVE OWNER, NUMBER OF SLAVES
George Williams, 17
http://www.angelfire.com/folk/gljmr/1860SimpsonS.html

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STRONG RIVER CEMETERY Located on Highway 472, approximately 2.5 miles from the intersection of Highway 472 and Highway 28. Established in 1828 "Those who honor the dead will care for the living".

Williams, George W. b 01 AUG 1819 d 22 MAY 1877
PLOT Row 11
" Age 57 years, 9 months, and 21 days".
http://www.geocities.ws/littleco/mastercm.html

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Pleasant A. Walker, 21, Private, Captain J. D. Funchess’ Co. (Westville Guards), Mississippi Vols.*, recorded as mustering into service at Westiville

* This company was subseqently designated as Company B, 16th Regiment Mississippi Infantry

Compiled Service Records of Confederate Soldiers Who Served in Organizations from the State of Mississippi