Recent works suggest that the increasing income inequalities decreased the middle class size and threatened life satisfaction (LS). However, no such studies link LS with middle-income class problems. The present study aims to fill this gap and characterize LS across the middle-income class in Poland. The main novelties are the Satisfaction with Life Scale (SWLS) to measure LS of the middle-income class in Poland and the methodological approach based on classification-regression trees (CART) to identify the determinants of LS. The analysis is conducted on the new database obtained from the survey of members of the middle-income class in Poland divided into three subgroups: the lower-middle-income class, the mid-income middle class, and the higher-middle-income class. For comparison, the low-income and high-income classes were also considered. Based on the SWLS results, relatively low diversity in middle-income class LS was identified. However, the inter-class comparison revealed that LS increased with belonging to a higher income class. The CART results support the primary importance of economic conditions such as, for instance, wealth and the level of savings in determining the middle-income class LS and show that their effect depends on their characteristics like professional status.