Hennessy, a pioneer in computer architecture, joined Stanford's faculty in 1977 as an assistant professor of electrical engineering. In 1981, he drew together researchers to focus on a technology known as RISC (Reduced Instruction Set Computer), which revolutionized computing by increasing performance while reducing costs. Hennessy helped transfer this technology to industry. In 1984, he cofounded MIPS Computer Systems, now MIPS Technologies, which designs microprocessors. He rose to full professorship in 1986 and was the inaugural Willard R. and Inez Kerr Bell Professor of Electrical Engineering and Computer Science (1987-2004). He has been director of Stanford's Computer Systems Laboratory (1983-1993); dean of the School of Engineering (1996-2000), and university provost (1999-2000). In October 2000, he was inaugurated as Stanford's 10th president. Throughout his career, Hennessy has multiplied the impact of Stanford's great minds by fostering interdisciplinary collaborations, initially in the biosciences and bioengineering and now among 18 interdisciplinary centers drawing from Stanford's seven schools. Upon stepping down from the presidency, he began serving as the inaugural Shriram Family Director of Knight-Hennessy Scholars. He also currently serves as chairman of Alphabet.
Hennessy shared the 2017 Turing Award with Dave Patterson (UC Berkeley) for "pioneering a systematic, quantitative approach to the design and evaluation of computer architectures with enduring impact on the microprocessor industry." Both Hennessy and Patterson's original work (on MIPS and RISC, respectively) was funded through the DARPA VLSI Program in the 1980s.
His honors include the 2012 Medal of Honor of the Institute of Electrical and Electronics Engineers, its highest award; the 2000 Benjamin Garver Lamme Award of the American Society of Electrical Engineers; the 2001 Eckert-Mauchly Award of the Association for Computing Machinery; the 2001 Seymour Cray Computer Engineering Award; a 2004 NEC C&C Prize for lifetime achievement in computer science and engineering, and a 2005 Founders Award from the American Academy of Arts and Sciences. Hennessy earned his bachelor's degree in electrical engineering from Villanova University and his master's and doctoral degrees in computer science from the State University of New York at Stony Brook.