This study investigated how adolescents’ Big Five personality traits are related to mental health (i.e., happiness) and mental illness (i.e., depressive symptoms and anxiety) and whether their time perspectives may be potential mediators of these relations. The sample included 235 high school students (54.5% girls), aged between 15 and 18 (M = 16.87, SD = 0.68). The results indicated that adolescents with higher emotional stability also reported higher levels of happiness and lower levels of depressive symptoms and anxiety, whereas those with high levels of extraversion or conscientiousness experienced higher happiness. Further, adolescents with low levels of past negative or high levels of past positive time perspectives also reported high levels of happiness and low levels of mental illness. Furthermore, high levels of present fatalist and low levels of present hedonist time perspectives were associated with low levels of happiness. Our findings provided evidence for the mediating role of the past negative and past positive time perspectives for the relations of extraversion, emotional stability and agreeableness with both mental health and mental illness. Finally, the present hedonistic time perspective mediated the relations of extraversion, agreeableness and conscientiousness with happiness, whereas the present fatalist time perspective mediated the relation between emotional stability and happiness. Our study highlights the importance of time perspectives as mediational factors for the relations of the personality traits with adolescents’ mental health and mental illness.