This study explores the relationship between tonal synchrony and maternal-infant social engagement based on free-play recordings of 15 mothers and their 3-month-old infants in a laboratory setting. Moment-by-moment analyses on a microlevel were used to study social engagement and vocal interaction. We analysed and categorized 854 vocalization periods (mother-only vocalizations, tonal interaction periods, nontonal interaction periods, and mutual silence). Tonal synchrony was analysed in terms of harmonic and pentatonic series based on pitch frequency analyses. Social engagement was microanalyzed in terms of matched and mismatched engagement states. ANOVA-repeated measures revealed, most importantly, a significant relationship between TIPs and social interaction repair, which indicates the importance of tonal synchrony in the flow of social engagement in mother–infant dyads. Other significant relationships were found between (a) nTIPs/mismatch–mismatch, and, (b) MOV/affect loss. As mentioned in the discussion, the findings are suggestive for clinical applications (e.g., music therapy) and warrant further research.