Background: The aim of this study was to compare health workers knowledge and skills competencies between those trained using the onsite simulation-based, low-dose, high frequency (LDHF) training plus mobile (m) mentoring and the ones trained using the traditional offsite, group-based training (TRAD) approach in Kogi and Ebonyi states, Nigeria, over a 12-month period. Methods: A prospective cluster randomized controlled trial was conducted by enrolling 299 health workers in 60 health facilities in Kogi and Ebonyi states, randomized to either LDHF/m-mentoring (intervention, n=30 facilities) or traditional group-based training (TRAD, n=30 facilities) control arm. These health workers in both arms received basic emergency obstetric and newborn care training with simulated practice using anatomic models and role-plays. The control arm was trained offsite while the intervention arm was trained onsite where they worked. Mentorship was done through telephone calls and reminder text messages. The multiple choice questions and objective structured clinical examinations mean scores were compared; p-value <0.05 was considered statistically significant. Qualitative data were collected and content analysis was done. Results: The mean knowledge scores between the two arms at months 3 and 12 post-training were equally high; no statistically significant differences. Both arms showed improvements in composite scores for assessed BEmONC clinical skills from around 30% at baseline to 75% and above at endline (p <0.05). Overall, the observed improvement and retention of skills was higher in intervention arm compared to the control arm at 12 months post-training, (p<0.05). Some LDHF/m-mentoring approach trainees reported that mentors’ support improved their acquisition and maintenance of knowledge and skills, which may have led to reductions in maternal and newborn deaths in their facilities. Conclusion: The LDHF/m-mentoring intervention is more effective than TRAD approach in improcquisition and retention. Health care managers should have the option to select the LDHF/m-mentoring learning approach, depending on their country’s priorities or context, as it ensures health workers remain in their place of work during training events thus less disruption to service delivery.