While interest in career optimism has grown steadily in the last 15 years, most of its investigations rely on cross-sectional convenience samples that prevent exploring how earlier optimism matters for later educational and vocational outcomes. To address this issue, I use structural equation modelling to assess measurement invariance and the impact of career optimism at ages 20, 23 and 26 in a nationally representative sample of 2900 young Australians. A three-item measure of career optimism shows metric but not scalar invariance across young adulthood, meaning that while optimism's components remain the same, their levels fluctuate. Optimism helps young adults enrol in a university and enter professional employment by age 26. However, the positive impact of career optimism partly reflects the variation in youths' socioeconomic backgrounds. Thus, future studies of career optimism should consider if some individuals tend to be more optimistic because they grew up in environments with more cultural and economic capital.