Ancestry.com confirms what you have mentioned and shows a William H. Menefee listed on the 1850 Census, living in Rappahonnock, Virginia with a birth date of about 1833. The 1850 slave census shows him as the owner of a black female, age 35.

An 1880 Census shows a man of that same name born about 1834, single, white male and living in Vellejo, Sonoma, California - born in Michigan - occupation, Carpenter - Parents born in Ireland.

Civil War records at Ancestry.com show two men of this name:

Name: William H. Menefee
Side: Confederate
Regiment State/Origin: Texas
Regiment Name: Mann's Reg't Texas Cavalry
Regiment Name Expanded: Mann's Regiment, Texas Cavalry (Bradford's)
Company: K
Rank In: Private
Rank In Expanded: Private
Rank Out: Private
Rank Out Expanded: Private
Film Number: M227 roll 25

Mann's Regiment, Texas Cavalry (Bradford's)

Mann's Cavalry Regiment, formerly Bradford's Cavalry Regiment, was organized during January, 1865. This unit served in the defense of Galveston and in March contained 29 officers and 361 men. It was included in the surrender on June 2. The field officers were Colonel Walter E. Mann, Lieutenant Colonel William F. Upton, and Major John E. Oliver.

William H. Menefee
Side: Confederate
Regiment State/Origin: Virginia
Regiment Name: 6 Virginia Cavalry
Regiment Name Expanded: 6th Regiment, Virginia Cavalry
Rank In: Private
Rank In Expanded: Private
Rank Out: Private
Rank Out Expanded: Private
Alternate Name: Henry W./Menefee, Jr.
Film Number: M382 roll 38

6th Regiment, Virginia Cavalry

6th Cavalry Regiment completed its organization in November, 1861, at Manassas, Virginia. Men of this unit were raised in Loudoun, Rappahannock, Clarke, Rockingham, Pittsylvania, Fairfax, Halifax, Fauquier, and Orange counties. The unit served in Robertson's, W.E. Jones', Lomax's, and Payne's Brigade, Army of Northern Virginia. It fought in Jackson's Valley Campaign and in the conflicts at Second Manassas, Brandy Station, Upperville, Fairfield, Bristoe, Mine Run, The Wilderness, Todd's Tavern, Spotsylvania, Haw's Shop, and Cold Harbor. The regiment went on to take part in Early's Shenandoah Valley operations and the Appomattox Campaign. Only 3 men surrendered on April 9, 1865, as most of the cavalry cut through the Federal lines and later disbanded. The field officers were Colonels Charles W. Field, Thomas S. Flourney, John S. Green, and Julien Harrison; Lieutenant Colonels J. Grattan Cabell and Daniel T. Richards; and Majors Cabell E. Flournoy and Daniel A. Grimsley.

Since Mann's Regiment Texas Cavalry did not form until January 1865, there is a possibility that this would be the same person as the one who served in the 6th Virginia Cavalry. The compiled military records on both men would be able to give you more clues.

Hope this helps.