Indonesia's current dominate generation, i.e., Millennials, has attracted a new marketing practice. This study aims to analyze Millennials' unique behavior and perform a market segmentation of respondents of two car brands from the perspective of the 7Ps marketing mix, personality, and lifestyle on Millennials' car buying decisions. For this purpose, the qualitative and quantitative research method was applied with in-depth interviews of eight respondents and deeper data collecting through snowball sampling for 300 respondents through offline and online surveys using questionnaires. Factor analysis identified 18 factors, which were further analyzed using cluster analysis. The cluster analysis revealed different cluster numbers for each car brand. There are three clusters in Brand H, namely, Need Everything Millennials, Basic Independent Millennials, and Emotionally Satisfied Millennials. Meanwhile, there are four clusters in Brand T: High Social Millennials, Simple-Minded Millennials, Basic Yet Ambitious Millennials, and Easy-Going Millennials. Significant differences were found between the groups in each brand in terms of decision-making roles, demographics, marketing mix considerations, personalities, and lifestyles. This study provides new insights regarding customer segmentation, especially for the Millennial generation in developing countries such as Indonesia.