What causes ineffective external idea evaluation in open innovation (OI) still remains an unsolved puzzle, with most such studies focused on creative idea generation or using samples of untrained evaluators. To help better understand the microfoundations of OI, this article examines the effects of mood on external idea evaluation using a practitioner sample. Drawing on “mood-as-an-input” theory, in two behavioral experiments using music induction, cognitive tasks, and idea framing, we test how one's mood affects the innovativeness rating of an externally developed idea, and examine whether this effect is stable within a mood state regardless of the level of creativity (high and low) of an idea. We found that people in happy and sad mood conditions differ in their evaluation of the same external idea, which is explained by differences in assessment of creativity of an idea and not the perceived certainty of its success. Moreover, a given mood state does not affect how ideas low in creativity are rated in their innovativeness, compared to ideas high in creativity. This article by investigating effects of mood within an OI process augments individual level OI literature, while informing the ways external idea evaluation can be managed toward enhancing OI potential.