Objective: Describing the long-term mental health of Australians is limited as many reports rely on cross-sectional studies which fail to account for within-person changes and age-related developmental processes which may bias estimates which ignore these phenomena. We examined the 17-year trajectories of mental health in 27,519 Australian adults. Methods: Household panel data of 27,519 participants aged 18 years and over from the Household, Income and Labour Dynamics in Australia Survey provided at least one observation of mental health over a 17-year period from 2001. On average, participants reported 7.6 observations. Mental Health was assessed annually using the Short-Form Health Survey-36 mental health scale. Results: Over time, there were only very small changes in mental health and only for the youngest and oldest adults. Over time, there was consistent evidence for better metal health with increasing age, although for the very old, there appear to be substantial declines. These patterns were consistent between sex. In line with an existing literature, males reported better mental health over life span, although the declines of mental health in very-late-life are particularly pronounced for males. Conclusion: Decline in mental health was only reported by the youngest and oldest respondents, and was notable only in the last 4–5 years. However, the magnitude of the decline was small and further follow-up will be needed to determine whether this is a trend of substantive declining mental health for these specific age cohorts. In contrast, the more consistent finding is that there has been no substantive change in the level of mental health in Australia over the last 17 years. Analysis of the mental health trajectories of baseline age-cohorts confirmed that age differences are consistent over time.