This article is Part 1 of a two-part series, in which we report on our evaluation of a mindfulness-based relationship enhancement program for couples who are expecting their first child. In this mixed-methods randomized clinical trial, we assigned 33 couples to the 4-week Mindful Transition to Parenthood Program treatment group (n = 16 couples) or to a waitlist control condition (n = 17 couples). Men in the treatment group significantly improved in relationship satisfaction, mindfulness, and negative affect; women had no significant treatment effects. Small to large effect sizes were present for treatment group men and women in multiple areas. Mixed-methods analyses demonstrated that this intervention may be especially helpful for men because of differences in social support needs, effects of program enrollment, and relational processes in the prenatal period.