Advantaged group members can play a significant role in advocating for social change on behalf of disadvantaged groups. Drawing on Kelley's covariation model and the attribution-identity model of ally sincerity, we argue that allyship that is inconsistent over time raises doubt about advantaged group members' motives. In four studies conducted in theUnited States, we found that Black Americans (Study 1, N = 330) perceived White Americans who participated consistently in protests for racial equality as more internally motivated and as better allies than those protesting for the first time, and that LGBTQIA+ individuals viewed inconsistent (i.e., decreasing or alternating) participation over time by straight allies as less internally motivated and less trustworthy than consistent participation (Study 2, 3, and 4, Ns = 287, 601, 434) or a control condition (Study 3). Our results highlight the importance of sustained allyship for positive perceptions of advantaged group allies.