This article is the second part of a comprehensive study on the rise of America’s early computer clusters from the perspective of evolutionary economic geography. That is, we attempt to identify the historical processes that led to the formation of the early clusters of computer production in the United States. Here we explore explanations for the genesis of the agglomerations identified in part 1. We suggest that the clusters we observe in the early 1970s arose from the locational decisions of three waves of companies: the computer start-ups of the late 1940s and early 1950s, the large conglomerates that entered the computer industry during the 1950s, and the minicomputer and peripheral-making companies that entered later on. We argue that a combination of preexisting conditions and Marshallian externalities generated the clustering patterns for computer production we observe in the early 1970s.