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.