The current study aimed to examine differences and associations of physical activity (PA) levels, quality of life (QoL), motives for PA, and stages of change (SOC) between young and mid adults. A total of 208 participants in Malaysia completed an online survey which comprised of questionnaires about PA, motivational factors, QoL, and SOC in PA. Independent t-tests revealed that young adults engaged in more PA in general, as well as light PA. However, mid adults in general scored higher levels of QoL and its four domains (i.e., physical, psychological, social, and environmental). Moreover, young adults were significantly more motivated by appearance for PA. Pearson correlations revealed higher levels of vigorous PA linked to, higher social domain of QoL in both age groups, and, physical and psychological domain of QoL in mid adults. Vigorous PA also showed the strongest positive links with motivations in competence in both young and mid adults. Cross tabulations showed that both young and mid adults have highest frequency counts in contemplation stage for moderate and light PA and highest frequency counts in the maintenance stage for vigorous PA. This study is the first to show how young and mid adults differed in PA levels, QoL, motives behind PA engagements, and how they correlate differently. However, both age groups showed similar SOC patterns, indicating the likelihood of moderate and light PA engagements when they see the possibility of change but still lack commitment, whilst those engaged in vigorous PA are more likely to actively maintaining the change.