William Overall Yager, banker, politician, and Confederate officer, was born in Luray, Virginia, on April 3, 1833, to Nicholas Wesley and Christina (Overall) Yager. Yager was raised and educated in Virginia and attended Virginia Military Institute and graduated fifth in the class of 1852. After graduation, he worked in Virginia briefly as a banker before moving to Texas, where he settled in Seguin, Guadalupe County. On June 23, 1863, Yager married Mary Elizabeth Rhodes in Seguin, and the couple had one son and four daughters.
When the Civil War began, Yager volunteered for service in the Confederate Army. On April 15, 1861, he was appointed as adjutant and major for the First Regiment, Texas Mounted Rifles, also known as McCulloch's Regiment. He spent the autumn and winter of 1861 with this unit in Central Texas and engaged in sporadic negotiations and skirmishes with local Indian groups. In April 1862, the First Regiment was reduced to a battalion of five companies and re-designated the Eighth Texas Cavalry Battalion. In May 1862 Yager was authorized to form a new cavalry battalion. This unit, when organized, was designated the Third Texas Cavalry Battalion. The Eighth and Third Texas Cavalry Battalions were consolidated on May 2, 1863, and re-named the First Texas Cavalry Regiment with Col. Augustus Buchel in command. On that same day, Yager was promoted to the rank of lieutenant colonel. In this final organization, Yager saw action in several engagements, including the battles of Mansfield, Pleasant Hill, Peach Orchard, and Yellow Bayou. After regimental commander Augustus Buchel's death at the battle of Pleasant Hill in April 1864, Yager was promoted to colonel and given command of the First Texas Cavalry Battalion, which he maintained until the end of the war.
Yager remained in Texas for some time after the close of the conflict, but by the 1870s, he returned to Virginia, where he represented Page County in the Virginia House of Delegates during the 1874–75 and 1879–80 terms. Yager also served as superintendent of schools for Page County. He died on January 20, 1904, and is buried at Yager Cemetery near Luray.