There is a need for effective and accessible web-based intervention strategy to promote positive parenting at a population level. We tested the effectiveness of the “Educar en Positivo” (“The Positive Parent”) program by the self-assignment of 148 Spanish-speaking parents to the modules (program group) and 164 parents to multimedia complementary material (visitor group). Both groups were automatically monitored for their daily parenting mood and online parental support via a standard questionnaire. Overall, the program spontaneously attracted mainly highly educated young mothers with young children. Results of a linear growth modeling showed a sustained progression in the daily mood measures of parental role satisfaction and confidence in parenting capacities in the program group. Pretest-posttest comparison in both groups showed significant improvements in the two types of measures only in the program group. Cluster analyses showed improvements in the daily mood but not in the standard measures of online support in 40% of participants (Clusters 1 and 2) as well as improvements involving a greater coherence across these measures in 60% of participants (Clusters 3 and 4). Participants in Clusters 3 and 4 reported greater satisfaction than participants in Clusters 1 and 2 with the program’s usability, content, and parenting impact. These results were modulated by the parents’ greater forum participation, use of a diary, and family-related commitments. The existence of asynchronies in the learning process and the relevance of implementation factors have research implications for the evaluation of universal online parenting programs.