As a graduate student at Stanford, he corresponded with APL's inventor, Ken Iverson, to correct the formal description of the IBM System/360 which used Iverson's notation. After receiving his M.S. from Stanford in 1965, under academic supervisor Niklaus Wirth,, An interpreter for Iverson notation he joined Iverson's group at IBM's Thomas J. Watson Research Center in Yorktown Heights, New York, where he created the first implementation of APL, with Philip S. Abrams, on a mainframe computer, an IBM 7090, in 1965. Obituary for Kenneth Iverson, Mathematician, 1920–2004, Monday, October 25, 2004, in the Toronto Globe and Mail The Socio-Technical Beginnings of APL, by Eugene McDonnell
Breed was the 1973 recipient (with Dick Lathwell and Roger Moore) of the Grace Murray Hopper Award from the Association for Computing Machinery "for their work in the design and implementation of APL\360, setting new standards in simplicity, efficiency, reliability and response time for interactive systems."
With Dan Dyer and others he co-founded Scientific Time Sharing Corporation in 1969, where he led the development of the APL PLUS time-sharing system. While there, in 1972, he and Francis Bates III wrote one of the world's first worldwide email systems, named Mailbox. APL Quotations and Anecdotes, including Leslie Goldsmith's story of the Mailbox
Breed rejoined IBM in 1977. He helped develop the International Organization for Standardization (ISO) APL standard, then joined IBM efforts to port Berkeley Software Distribution (BSD) Unix onto IBM platforms. He worked on for the programming language C, floating-point arithmetic standardization, and radix conversion, until retiring in 1992.
In 1973 and 1974 he took first place, with co-solver Donna Breed, in the Dictionary Rally.