Abstract Objective Internet-based interventions may positively impact maternal symptoms of postnatal depression and anxiety. This study assessed the feasibility, acceptability, perceived usefulness, and preliminary effectiveness of an m-Health version of “What Were We Thinking?” (m-WWWT). Methods A mixed-methods with a 2-arm randomized parallel design was used. From a total of 477 women, 157 met the inclusion criteria. 128 first-time mothers of full-term infants, aged 4–10 weeks, who received health care at primary public health centers in Chile, were randomly assigned to the experimental (EG, n = 65) or control (CG, n = 63) groups; data of 104 of them (53 and 51, respectively) was analyzed. We used percentages and rates to measure feasibility outcomes and mixed analysis of variance (ANOVA) and latent class analyses (LCA) to assess preliminary effectiveness. Participants completed questionnaires on mental health, social support, and maternal self-efficacy upon recruitment and 3 months after completing the intervention. For the qualitative component, 12 women from the EG were interviewed. Results Quantitative results show good feasibility outcomes, such as high recruitment (82%), low attrition (EG = 12% and CG = 17%), and high follow-up (EG = 97% and CG = 91%) rates. Qualitative results indicate high acceptability and perceived usefulness of m-WWWT. Mixed ANOVA did not show significant differences between the groups (all p >.05). However, multinomial regression analysis in LCA showed that women with low baseline symptoms of depression and anxiety benefit from the intervention (B = 0.43, 95% confidence interval 1.09–2.16). Conclusion m-WWWT is feasible to be implemented in Chile; future studies are needed to assess the intervention’s effectiveness.