This study examined the synthesis of alumni investment schemes and academic service sustainability in public universities in the South-South Zone of Nigeria, focusing on the structure of alumni investment schemes, their contribution to academic service sustainability, and the institutional factors influencing their effectiveness. A descriptive survey research design was adopted. The target population comprised all administrative and academic staff in public universities in the South-South Zone of Nigeria, including Vice Chancellors, Registrars, Bursars, Deans, Heads of Departments, Directors of Alumni Relations, student affairs officers, and senior academic staff involved in institutional development and academic service delivery. From this population, a sample of 350 respondents was drawn using a multi-stage sampling technique. In the first stage, the South-South Zone was stratified according to states. In the second stage, public universities were randomly selected from each state. In the third stage, respondents were proportionally selected from the sampled universities using stratified random sampling based on staff role categories (administrative staff and academic staff) to ensure balanced representation of perspectives on alumni investment schemes and academic service sustainability, thereby enhancing the representativeness, reliability, and generalizability of the findings. Data were collected using a researcher-developed instrument titled Alumni Investment Schemes and Academic Service Sustainability Questionnaire (AIS-ASSQ) and analyzed using mean, standard deviation, t-test, and ANOVA at the 0.05 level of significance. The findings revealed that alumni investment schemes in public universities are weakly structured and unevenly institutionalized, their contributions to academic service sustainability are positive but modest and insufficient as standalone funding mechanisms, and institutional factors such as leadership commitment, governance quality, transparency, trust, and monitoring systems significantly influence their effectiveness. The study concludes that alumni investment schemes function primarily as complementary funding mechanisms rather than sustainable primary funding structures, and that their impact is largely determined by institutional capacity and governance conditions. It therefore recommends the institutionalization of alumni investment frameworks, integration of alumni funding into national university financing policies, and the strengthening of governance, accountability, and monitoring systems to enhance sustainable academic service delivery in Nigerian public universities.