Non-Governmental Organizations (NGOs) serve a pivotal function in ensuring the well-being of the people they serve, and this is mostly the underserved populations. The general aim of NGOs is to create a favourable balance between people while upholding the value of humanity and equity. Despite this crucial role, most NGOs, especially those found in informal settlements such as Kibera are grappling with financial sustainability. Given this case, most of these entities are in the brink of closure, with some of them being unable to effectively meet their mission, and this is blamed on poor financial management practices. This research sought to ascertain the effect of financial management practices on the financial sustainability of registered NGOs operating in informal settlements. Specifically, it aimed to assess the influence of financial planning on financial sustainability of NGOs in Kibera. The research was anchored on two theoretical frameworks: the Contingency Theory and Agency theory. The study utilized the descriptive design. The research targeted 150 finance officers, from the 150 registered NGOs in Kibera. Stratified sampling approach was employed to arrive at 112 respondents. Primary data was obtained utilizing a structured questionnaire, which was distributed and retrieved using the drop-and-pick method. The collected responses was analyzed using SPSS version 29, where descriptive and inferential statistics was carried. Descriptive statistics entailed frequency distributions, means and standard deviations while inferential statistics entailed Pearson moment correlation and establishing the relation of the study variables using a multiple regression model. Diagnostic tests was carried out before the actual data analysis (normality tests, multicollinearity tests and heteroscedasiticity tests). The findings of the analyzed data was presented through figures, tables, and charts. All the ethical issues pertaining research writing were adhered to. This research found that financial planning significantly impact financial sustainability of registered non-governmental organizations. The research recommends for the establishment of proper financial planning measures that serves as a roadmap, enabling businesses to allocate resources effectively, mitigate risks, and capitalize on opportunities to improve financial sustainability of registered non-governmental organizations