The aim of this longitudinal study was to investigate if social inequality in onset of mobility limitations is present in early midlife, and to examine whether common determinants of disability account for social status differences in onset of mobility limitations in midlife. 3,586 40-and 50-year-old Danish men and women enrolled in The Danish Longitudinal Study on Work, Unemployment and Health constituted the study population. Data were collected by mailed questionnaires in 2000 and 2006. Limitations in running 100 m and in climbing two flights of stairs represented two aspects of mobility limitations. Occupational social class was applied as a measure of socioeconomic position. Among individuals reporting no limitations in 2000 social gradients in onset of mobility limitations emerged in 2006. Having a low social class at age 40 and 50 respectively was associated with higher onset of mobility limitations 6 years later, and this association was overall only partly explained by common determinants of disability: smoking, alcohol consumption, physical activity, body mass index, and chronic diseases. These results indicate that the development of mobility limitations might begin earlier in lower compared to higher social strata, and that social status differences in mobility limitations might emerge already in the forties.