Participatory learning and action cycles with women’s groups have been recommended by the WHO to promote maternal and newborn health, but few studies have tested its feasibility and acceptability in mobile health (mHealth) interventions among mothers of toddlers. This was a mixed-method feasibility assessment of an 8-week WhatsApp-based maternal support group for mothers of toddlers (12–18 months of age) enrolled in a birth cohort study in Southern Brazil. Daily messages and weekly activities were sent by moderators to promote maternal–child outcomes: child nutrition, child sleep, nurturing care, and maternal psychosocial well-being (assessed pre- and post-intervention via self-reported questionnaire). The implementation and engagement of the mothers in the program were assessed by message extraction. Acceptability was evaluated through in-depth interviews (n = 5) and open-ended surveys (n = 10). 1481 messages were exchanged in 3 WhatsApp groups (n = 30 mothers). Mothers were most active on weekdays (68.6% of messages sent on Tuesdays and 72.6% on Thursdays), afternoons (2:00–4:00pm), and evenings (9:00–11:00 pm). Engagement was higher at weeks 1–4. Mothers enjoyed and considered topics relevant. Group interaction was perceived as low, which influenced their participation. The prevalence of depression symptoms decreased from pre- to post-intervention (9% to 5%; P =.04). A moderated mobile-based support group for mothers of toddlers was feasible. mHealth services to promote maternal support are a promising strategy to improve maternal–child outcomes, but engagement and use of the service remains a challenge. Program managers should work with community members to identify ways to support engagement and participation throughout the intervention.