The effects of working part time on job downgrading and upgrading are examined over the life course of British women born in 1958. We use longitudinal data with complete work histories from a large-scale nationally representative cohort study. Occupations were ranked by their hourly average earnings. Analyses show a strong link between full-time/part-time transitions and downward and upward occupational mobility over the course of up to thirty years of employment. Probabilities of occupational mobility were affected by women's personal traits, occupational characteristics and demand-side factors. Downward mobility on moving from full-time to part-time work was more likely for women at the top levels of the occupational hierarchy working in male-dominated or mixed occupations and less likely in higher occupations with more part-time jobs available.