Hennessy is Stanford University's 10th President. He came to Stanford in 1977, where he was the Willard and Inez Kerr Bell Professor of Electrical Engineering and Computer Science, Dean of the School of Engineering and Frederick E. Terman Professor of Engineering prior to his appointment as Provost in July 1999.

Hennessy received his B.E. in Electrical Engineering from Villanova University and a Masters (1975) and Ph.D. (1977) in Computer Science from SUNY at Stony Brook. He is the recipient of the 1983 John J. Gallen Memorial Award, awarded by Villanova to the most outstanding young engineering alumnus; a National Science Foundation Presidential Young Investigator Award; the Distinguished Alumnus Award from the State University of New York at Stony Brook,1991; and the 1994 IEEE Piore Award. He is an Institute of Electrical and Electronics Engineers (IEEE) Fellow, a member of the National Academy of Engineering, and a Fellow of the American Academy of Arts and Sciences and the Association for Computing Machinery.

In 1981, he initiated the MIPS project at Stanford; MIPS is a high-performance Reduced Instruction Set Computer (RISC). In addition to his role in the basic research, he played a key role in transferring this technology to industry. His current research interests are in building very high performance computers and in making such machines useful to a wide variety of potential users.