Entrepreneurship has become a worldwide phenomenon such that various government and non-profit organisations are developing different programme for encouraging start-up of new entrepreneurs for youth, middle aged and retired people (senior entrepreneurs). Supporting senior entrepreneurs is also emerging in developing countries and policy makers are trying to develop strategies to encourage start-up of new business. The purpose of this study was to examine the factors influencing senior entrepreneurs in starting up their business in Tanzania a case of Mwanza city. The study was guided by the following objectives: (i) to determine the types of business undertaken by senior entrepreneurs in Mwanza city, (ii) to identify the characteristics of senior entrepreneurs in Mwanza city, (iii) to determine different factors that influence senior entrepreneurs in starting up their businesses. The research employed both quantitative and qualitative methods within descriptive research design to answer the research objectives. The study surveyed 115 respondents using self administered questionnaires. In addition in depth interviews were conducted with the business licensing officers and the retirees. The data were analyzed using descriptive statistics through SPSS and Excel computer packages. The research finding revealed that retirees (senior entrepreneurs) are influenced by factors such as human capital, financial capital, access to information and regulations, social environment and social capital in starting up their business. When ranked in order of their importance, financial capital was ranked as the most important influential factor followed by human capital, access to information and regulations and last was social capital. The study recommends the following measures to be taken to improve the performance of businesses of retirees: prospective retirees must embrace retirement planning which starts with early saving for retirement. Retirees should endeavour to attend training seminars to improve their business and financial management skills. The prospective retirees should start business before retirement age to gain hands on experience and build foundation for their businesses well before retirement age. The study suggests areas for further research such as extensive studies in other regions prior for generalization and should focus on the internal and external factors affecting the performance of business of retirees in Tanzania.