In Ethiopia, quality of maternal and newborn care is poor. This situation has persisted, despite the wide implementation of several capacity building-oriented interventions including clinical mentoring for skilled birth attendants that were anticipated to translate in to high-quality maternal and newborn care on each encounter. The effectiveness of mentoring programs is not yet well documented in the research literature. Therefore, we evaluated the effect of a catchment based clinical mentorship in improving the quality of maternal and newborn care in primary level facilities of Tigray, Northern Ethiopia. We conducted a controlled quasi-experimental pre-post study among 19 primary health care facilities, with 10 facilities assigned to the group where the catchment based clinical mentorship program was implemented (intervention group), and 9 facilities to the control group. We assigned the group based on administrative criteria, number of deliveries in each facility, accessibility, and ease of implementation of the intervention. A sample of 1320 women(662 at baseline; 658 at post intervention) and 233 skilled birth attendants(121 at baseline and 112 at end line) were included. We collected data from mothers, skilled birth attendants and facilities. The first round of data collection (baseline) took place two weeks prior the inauguration of the intervention, 05 October to 04 November 2019. The end line data collection occurred from 22 May to 03 July 2020. The primary Outcome was "receipt quality of maternal/newborn care". We analyzed the data using difference in differences (DiD) and logistic regression with Generalized Estimating Equation. The level of significance of predictors was declared at p-value less than 0.05in the multivariable analysis. We deployed a team of local clinical mentors working at primary hospitals to provide clinical mentorship, and direct feedback in routine and emergency obstetrical and newborn care to the mentees (all skilled birth attendants performing maternal and newborn health services) functioning in their catchment rural health centers for duration of six months. While visiting a facility, mentors remain at the facility each lasting at least five to seven days per month, over the course of intervention period. A significantly higher proportion of women at intervention facilities received quality of care services, compared with women at comparison facilities. (DiD = 18.4%, p<0.001). Moreover, following the implementation of the intervention we detected a difference in the occurrences of maternal complication outcome during delivery and immediately after birth. This was decreased by 4.5%, with significant differences between intervention and comparison sites (DiD = 4.5%, p = 0.013). We also found a favorable difference in occurrences of neonatal obstetric complications, with a decrease of 4.8% in the intervention site and almost no change in the comparison site (DiD = 4.8%, p = 0.002). Among the determinants of quality of care, we found that providers' job satisfaction (AoR = 2.95, 95%CI: 1.26 to 6.91), and making case presentation at regular basis(AoR = 1.89, 95%CI: 1.05 to 3.39) were significantly associated to improve the quality of care. However, delivery load(AoR = 0.95, 95%CI: 0.93 to 0.98) was negatively associated with quality of care. We conclude that the catchment based clinical mentorship intervention is effective to improve quality of care and reduce childbirth complications in northern Ethiopia. This finding further elaborated that incorporating maternal and newborn health catchment based clinical mentorship activities into the existing health system strengthening strategies can catalyze improvement processes to quality practice and health systems. This is seen as a necessary step to achieve the effective quality universal health care required to meet the health-related Sustainable Development Goals. Besides, more attention needs to be given to develop interventions and strategies that directly enhance providers' job satisfaction and reduce delivery work load.