Wallach was the co-founder and CTO of Convey Computers. After Micron Technology bought Convey, Wallach became a design director. Wallach was previously Vice President of technology for Chiaro Networks and was co-founder of Convex Computer, their Chief Technology Officer and Senior V.P. of Development. After Hewlett-Packard bought Convex, Wallach became the chief technology officer of Hewlett-Packard's large systems group. He was also a visiting professor at Rice University from 1998-1999. Prior to Convex, he was manager of Advanced Development for Data General. His efforts on the MV/8000 are chronicled in Tracy Kidder's Pulitzer Prize winner The Soul of a New Machine. Prior to that, he was an engineer at Raytheon, where he worked on the All Applications Digital Computer (AADC). Wallach has 40 patents and is a member of the National Academy of Engineering, an IEEE Fellow and was a founding member of PITAC (The Presidential Information Technology Advisory Committee).

He is currently an adviser to Centerpoint Venture partners, Sevin Rosen Funds, and Interwest, and a consultant to the United States Department of Energy Advanced Scientific Computing (ASC) program at Los Alamos National Laboratory.