In January 2018, Fall was named the principal deputy director of the Advanced Research Projects Agency-Energy and has been serving as that agency's top official in the absence of a director. Prior to joining ARPA-E, he held a series of positions at the Office of Naval Research, including acting chief scientist. From 2014 to 2017, he was detailed to the White House Office of Science and Technology Policy as assistant director for defense programs and later as acting associate director for national security and international affairs.

Fall holds a bachelor's degree in mechanical engineering and a Ph.D. in neuroscience, both from the University of Virginia.