He was born at Longwood House in Farmville, Virginia and graduated from Hampden-Sydney College, founded by his Patriot ancestor Nathaniel Venable, at the age of 15.

Venable was present at the firing on Fort Sumter in April 1861, serving as a lieutenant in the South Carolina state militia.

In the spring of 1862, Venable joined the staff of presidential military advisor General Robert E. Lee as an aide-de-camp with the rank of major.

He continued serving on Lee's staff when the general took command of the Army of Northern Virginia on June 1, 1862.

He served on Lee's staff from the Peninsula Campaign to Appomattox Court House and was promoted to lieutenant colonel.