Gauteng was one of the provinces in South Africa most hit by COVID-19. However, there has been no assessment of the pandemic's impact on essential maternal, neonatal and child health (MNCH) services in Gauteng, for planning against future emergencies. This study sought to assess the impact of the COVID-19 pandemic on essential MNCH service utilisation, delivery and health outcomes in Gauteng province. We employed a quasi-experimental interrupted time series (ITS) study design, using the District Health Information System (DHIS) data set to evaluate the impact of COVID-19 on eight key MNCH indicators between March 2019 to February 2021. Using Stata V.17.0 and 5% alpha, a segmented linear regression (ITS) model quantified the trends of the indicators before COVID-19 (March 2019 to February 2020) (β1), the immediate change in level due to the March 2020 lockdown (β2), the post-lockdown (March 2020 to February 2021) trend (β4) and the change in gradient from before to after the lockdown (β3). COVID-19 lockdown exerted a significant decline in primary healthcare headcount<5 years (n) (β2= -60 106.9 (95% CI, -116 710.4; -3503.3), p=0.039); and postnatal care visits within 6 days (rate) (β2=-8.2 (95% CI, -12.4; -4.1), p=0.001). Antenatal care first visits before 20 weeks (rate) declined during COVID-19 (β3=-0.4 (95% CI, -0.7; -0.1), p=0.013) compared with the pre-COVID-19 period. COVID-19 adverse effects on service delivery (measles second dose coverage and fully immunised<1 year) and health outcomes (facility deaths 0-6 days, maternal mortality ratio and pneumonia case fatality<1 year) were insignificant. While some indicators post-lockdown attempted to recover, others deteriorated. In Gauteng province, the COVID-19 pandemic significantly disrupted essential MNCH service utilisation, particularly during the March 2020 lockdown. The mechanism of MNCH service disruption by COVID-19 was induced by both supply and demand services. It is imperative to strike a balance between maintaining routine healthcare services and managing an outbreak.