While at Bell Laboratories, he also attended Stanford University, where he earned a Ph.D. degree in Electrical Engineering in 1984, under the supervision of Thomas Kailath.

In 1984, Cioffi left Bell Labs to work at IBM as a hard disk drive read channel researcher.

[2] His and his students' research into discrete multitone modulation (DMT) became widely adopted in digital subscriber line (DSL) technology, used commonly for Internet access.

In 2001, Cioffi was elected as a member into the National Academy of Engineering for contributions to the theory and practice of high-speed digital communications.

In 2003, Cioffi founded Adaptive Spectrum and Signal Alignment, Inc. (ASSIA) to help service providers realize improvements in the performance and profitability of their DSL networks.