Before joining APL, Semmel held leadership and technical positions with
Wang Laboratories and the
MITRE Corporation. From 1997 to 2010, Semmel was chair of Computer Science, Information Assurance, and Information Systems Engineering graduate programs for Johns Hopkins University's
Whiting School of Engineering’s ‘Engineering for Professionals’ program. In 2017, Semmel was named an "International Business Leader" by the
World Trade Institute. In 2017, Semmel was a commencement speaker for the
University of Maryland, Baltimore and received an honorary Doctor of Science degree. On May 24, 2019, Semmel delivered the commencement speech and received an honorary associate of arts degree at
Howard Community College.
The Daily Record recognized Semmel as an "Influential Marylander" in an issue released March 29, 2019 and as a top 30 "Power in Higher Education" in 2022. Under Semmel's leadership, the Lab had a wide variety of accomplishments, including the successful
Pluto flyby of APL-built
New Horizons, the data modeling for the
Johns Hopkins Coronavirus Research Center and the successful
Double Asteroid Redirection Test, which was the world's first planetary defense test mission. Semmel's leadership also saw APL selected multiple times as one of
Fast Company's Best Workplaces for Innovators and
ComputerWorld's Top 10 Best Places to Work in IT. In July 2024, Semmel announced that he would be stepping down from his role as director of APL in 2025, after almost 40 years at the Lab. He is the second-longest-serving head of the Lab, trailing only its third director, Ralph Gibson. In 2026, Semmel was elected as a member of the
National Academy of Engineering. == Research ==