Loneliness has potentially adverse effects on health and is often connected with depression, while maternal depression has been shown to have a harmful effect on many indicators of children's socioemotional outcomes. The prevalence of loneliness among first-time mothers and its associations with depressive symptoms, background factors, and child outcomes in middle childhood and adolescence were investigated in this longitudinal study. A sample of 122 mother-child dyads was collected from maternal health clinics in Tampere, Finland. The mothers reported their feelings of loneliness at three time points: during pregnancy and when their firstborn child was 8-9 and 16-17 years old. Maternal depressive symptoms were screened using the Edinburgh Postnatal Depression Scale (EPDS) at the same time points. The children's internalizing and externalizing problems were assessed using the Child Behavior Checklist (CBCL) and Youth Self Report (YSR) questionnaires. Some 34-38% of the mothers reported loneliness at the different study points. Maternal loneliness showed associations with dissatisfaction with life and the pair relationship, and with the presence of depressive symptoms. The mother's prenatal loneliness predicted the child's internalizing problems in adolescence. Interventions aimed at relieving loneliness should be provided for mothers at all stages of motherhood.