In this study, we discuss how to best use alumni networks for entrepreneurial and incubation processes of university spin-offs capitalizing on technology seeds. On the basis of previous studies, findings and theories, we hypothesized that alumni networks would be successful platforms through which mentors could support, advise, guide, and/or supervise entrepreneurs. To prove this claim, we conducted (1) a survey of incubation offices and alumni liaison offices in universities across Japan and (2) case studies of three advanced mentor organizations operated as alumni networks. The findings validated the hypothesis and further identified (1) the crucial characteristics of alumni networks for university spin-off development and (2) that a common premise and objective for the alumni network and the heterogeneity of its members, realized by including a certain percentage of alumni from other universities, is indispensable for a successful mentor platform.