We completed a retrospective multivariate panel and longitudinal trend study to evaluate the effect of Gavi Vaccine Alliance grants on vaccine-preventable disease (VPD) post-neonatal mortality. Feasible Generalized Least Squares (fGLS) regression analysis was used to examine the association between VPD post-neonatal mortality rates and Gavi funding. We also applied segmented regression analysis to assess the structural trend in VPD post-neonatal mortality rates, the impact of Gavi funding, and trend changes following Gavi support. We separately tested a composite VPD mortality rate and five vaccine-preventable mortality rates: pertussis, meningitis, measles, diarrhea, and pneumonia (lower-respiratory infection) as dependent variables. All 77 countries eligible for Gavi assistance from 2000 to 2014 were included in the study. To isolate the effect of Gavi funding in our primary model, we controlled for known and likely predictors of child mortality. Among other factors, Gavi investment and corruption control/system efficiency are important elements to reduce vaccine-preventable mortality rates. For every $1 per capita invested by the Gavi Vaccine Alliance, there are highly statistically significant effects- decreasing the vaccine-preventable disease post-neonatal mortality rate by 2.535 per 1,000 live births. We also found Gavi investments to be significantly associated with reductions in four vaccine preventable disease-specific rates: pertussis, meningitis, diarrhea, and pneumonia. Furthermore, we found Gavi support to be significantly associated with both immediate impacts and trend changes on vaccine-preventable mortality rates. We conclude that Gavi investments in developing country immunization programs have measurably contributed to reductions in post-neonatal VPD mortality rates. Furthermore, we found evidence of a longer term catalytic effect of Gavi funding with accelerated reductions in the trend for post-neonatal meningitis, diarrhea, and pneumonia mortality rates.