Background and Objective: The number of zero-dose and under-vaccinated children has passed 20 million following the Covid-19 pandemic. Reducing missed opportunities for vaccination is recommended as a key strategy for increasing coverage because it involves utilising existing vaccination sites. To generate actionable evidence for coverage and equality improvement targeting, this study aimed to estimate the frequencies of missed opportunities for simultaneous vaccination (MOSVs) and their correction by residential area and assess the impact of MOSVs on full vaccination and coverage of vaccine doses in The Gambia. Methods: Data of children with cards aged 12–23 months from The Gambia 2019/2020 demographic and health survey was used (weighted n = 1355). We measured the number of children who experienced at least 1 MOSV any time before the survey and the proportions of children who later 1) received all doses, 2) received some doses, and 3) never received any dose by residence. Finally, valid coverage with and without MOSVs was estimated for all eligible vaccine doses. Results: More than half of the children surveyed experienced at least one MOSV, and more than half of the MOSVs were later corrected. A quarter of the children who experienced MOSVs did not have them corrected. Rural and urban residents had similar experiences in the proportion of MOSVs, but children in urban areas had their MOSVs not corrected more frequently. Seventeen of eighteen vaccine doses in the national schedule would have coverage gains without MOSVs, with some gaining as much as nine percentage points in coverage. Conclusions: Missed opportunities for simultaneous vaccination are frequent, negatively affecting coverage and differentially impacting rural and urban areas, and should be targeted for vaccination improvement. Our study emphasizes the importance of MOSVs for vaccination coverage and the need to implement the WHO missed opportunities for vaccination strategy.