The western terminus of FM 122 is at an intersection with SH 207. The route travels east along Tilford Avenue. The FM 122 designation and state maintenance end at an intersection with Avenue G; at this point, the roadway turns from pavement to brick.
FM 122 was originally designated in 1945, as a much longer north–south route that replaced a section of SH 207 from Ralls southward to Caprock. Another segment was added later that year, from Post to the north approximately 8 miles, creating a gap in the route's mileage. The FM 122 designation was applied to the roadway between Post and Caprock in 1949, removing the gap. In 1958, a spur was designated in Ralls. This spur is currently what remains of FM 122; the portion from Ralls to Post was redesignated as part of SH 207 as a part of that route's lengthening in 1967.