This research is about the Sustainability Index calculations of Pakistan's microfinance institutions and identifying those factors, which significantly affect the Sustainability Index. The analysis is employed on the sample of Pakistan's microfinance institutions for 2020 using the TOPSIS method for sustainability index and regression analysis for contributory factors. The sustainability score for MFIs ranges from a maximum score of 0.836 to a minimum of 0.005. Gross Loan Portfolio, Number of Borrowers per Staff Member, and Return on Assets are significant contributors to sustainability scores of Pakistani MFIs. By focusing on these contributory factors, policymakers and regulators may devise policies to support the long-term sustainability of MFIs. These contributory factors are not only useful to determine the sustainability of MFIs but can also assist the future researchers in unfolding further factors of sustainability.