Background: Both the symptoms and diagnoses of mental health disorders affect individuals’ occupational status and income. However, studies that compare the impact of differences between symptoms and diagnoses on occupational outcomes are lacking. Aims: This study aimed to compare labour market attachment and income until midlife between individuals with different histories of mental disorder symptoms and diagnoses. Method: Utilizing the Northern Finland Birth Cohort 1966 with linkages to national registers and self-reported mental disorder symptoms at the age of 31, we compared labour market attachment and income until midlife among individuals with neither mental disorder symptoms nor a diagnosis (reference group), symptomatic undiagnosed mental disorder, asymptomatic diagnosed mental disorder, and with symptomatic diagnosed mental disorder by using cross-tabulations and regression analyses. We stratified our analysis by sex. Results: Compared to the reference group, males but not females with symptomatic undiagnosed mental disorder had an increased risk for poor labour market attachment, with Odds Ratios (95% Confidence intervals) 2.26 [1.41, 3.63] and 0.87 [0.63, 1.19], respectively. The analogous risk was heightened for both males and females with asymptomatic diagnosed mental disorders or symptomatic diagnosed mental disorders. Regarding income, having mental disorder symptoms, a diagnosis, or both was associated with lower earnings irrespective of sex. Conclusions: Mental disorder symptoms and diagnoses are differently associated with labour market attachment and income. Our findings suggest that interventions should be offered not only based on diagnoses but also based on symptoms as they may serve as predictors of future challenges related to work careers.