This study examines the contributions of Nigerians in the diaspora to the development of Nigeria between 1999 and 2019. It focuses on political, economic, and socio-cultural dimensions of diaspora engagement and evaluates how remittances, knowledge transfer, and international advocacy have influenced national development. The paper adopts a descriptive and analytical approach, drawing from secondary sources including scholarly articles, institutional reports, and policy documents. Findings reveal that diaspora Nigerians play a significant role in foreign exchange earnings through remittances, capacity building through skills transfer, and political development through advocacy and participation in democratic processes. However, structural barriers such as weak institutional frameworks, lack of diaspora engagement policies, and corruption have limited optimal utilization of diaspora potential. The study concludes that strategic diaspora engagement is essential for sustainable development in Nigeria.