Pennybacker worked for the Texas Highway Department in the early 1900s designing bridges. He earned his civil engineering degree from the University of Texas at Austin. He served as a captain in the Army Air Service during World War I. After the war, he worked in Kansas and Texas. During World War II, he became interested in welded construction as an alternative to rivets. By promoting the use of welding for heavy stress bridge design, he is credited with saving the state of Texas millions of dollars.