Extant literature reveals that many e-government initiatives fail especially in developing countries. Despite the alarming failure rate of e-government initiatives, governments of developing countries are allocating a huge budget to support and improve e-government services. Prior research suggested that maturity is a prerequisite for e-government sustainability, but the relationship between e-government maturity and sustainability is not investigated. Hence, this research is aimed at developing a conceptual framework that maps e-government maturity to a sustainable e-government service. First, the determinants of e-government maturity and sustainability were derived from the extant literature. Then, the proposed conceptual framework is revised based on qualitative research using a multiple exploratory case study considering four G2C cases selected from the e-government portal of Ethiopia. The conceptual framework was evaluated by domain experts, and the empirical data were analyzed using the thematic analysis technique. The study results show that manager's commitment to prioritize e-government projects, availability of an ICT department with an independent annual budget for the e-government development, human resource capability, degree and frequency of customer contacts, ICT infrastructure development, integration, website age, legal and political strategies, organizational e-government operational plan, and e-payments were identified to be the determinants of e-government maturity. The study contributes to e-government literature by providing a better understanding of the determinants of e-government service maturity and sustainability and the link between the two constructs. The outcome of the research could also be of value for practitioners as a quality tool to assess the maturity and sustainability of e-government initiatives. Evaluating the applicability of the proposed framework in different country contexts, a cross-case study across nations, and conducting a longitudinal investigation of e-government maturity and sustainability are avenues for future research.