MD 135 was originally built in the early 1930s as a link between McCoole and Westernport. In the late 1930s, the state highway was extended west to Bloomington. In addition, a separate segment of MD 135 was completed around Swanton in the mid-1930s. These separate segments were unified in the early 1950s when the highway over Backbone Mountain was completed. In the late 1950s, MD 135 was extended west over a portion of MD 38 and all of Maryland Route 41 to Oakland, and was mostly rebuilt over its entire length, bypassing Bloomington, Swanton, Deer Park, and Mountain Lake Park. MD 135 remains notorious for the eastbound descent from Backbone Mountain, which has claimed many lives. The Maryland State Highway Administration has instituted many measures to get trucks down the mountain safely and to avoid the downgrade if possible.