Introduction: A woman’s nutrition during pregnancy and nursing affects the mother and the growing child. Similarly, the first two years of a child’s life are critical to their growth and development and are facilitated by optimum nutrition. Women’s nutrition-related knowledge, attitudes, and practices influence household food and nutrition security. Mobile health (mHealth) is a potentially effective health intervention in pandemic situations when physical gatherings are restricted. Objectives: To examine the effectiveness of a mobile phone-based nutrition education intervention targeting pregnant and nursing mothers in six Sri Lankan divisional secretariat areas. Method: This intervention was evaluated using a before and after within-subjects design. The intervention included 19 messages over four weeks sent via mobile phone, covering nutrition themes such as pregnancy care, infant and young child-feeding, diet, family care for mother and child, and cash management. The intervention was evaluated based on a quantitative survey using a structured interviewer-administered questionnaire and qualitative interviews using a semi-structured questionnaire. The study population was pregnant and nursing mothers. The objective of the qualitative interviews was to identify how respondents used messages and how satisfied they were with the project. The outcome measures were awareness/knowledge, attitudes, social norms, self-efficacy, behaviour intentions, and practices of pregnant and nursing mothers. Trained enumerators collected data using a mobile phone. Results: A total of 996 pregnant and nursing mothers participated in the pre-assessment survey, of which 720 completed the post-assessment. Most were nursing mothers (84.2% pre- and 78.9% post-assessment). Participants provided positive feedback on the intervention. Knowledge/awareness (t = −18.70, p < 0.01) and attitudes (t = −2.00, p < 0.05) increased when exposed to the intervention. Favourable improvements in the practices were also observed. Mothers’ practices related to breastfeeding and 24-h dietary diversity showed a statistically significant improvement. However, social norms and behaviour intentions did not significantly improve. The qualitative component also revealed favourable responses. Conclusion and Recommendations: The mobile intervention improved participants’ knowledge, awareness, attitude, and practices, but not social norms or behaviour intentions. This approach is recommended to be used on a larger scale in community settings. In addition, mobile technology could drive intervention in pandemic-related situations.