Background: Loneliness is associated with poor functional ability in older people. Little is known about this association in the middle-aged. The aim is to investigate if perceived loneliness is associated with lower physical capability among middle-aged men and women and if the associations of loneliness with physical capability interact with socioeconomic position and cohabitation status. Methods: 5224 participants from Copenhagen Aging and Midlife Biobank (CAMB) aged 49–62 years (mean age 54) were included. Handgrip strength (measured by a dynamometer) and maximal number of chair rises in 30 s was recorded. Multivariate linear regression analyses were adjusted for age, occupational social class, cohabitation status, morbidity and personality traits. Results: No association was found between loneliness and physical capability. For example estimates for handgrip strength in ‘often’ lonely men and women compared with the ‘not lonely’ were 1.2 kg (95% CI − 0.5;2.9)/1.0 kg (−0.7;2.6). Low occupational social class was associated with poorer physical capability, and living alone was associated with poorer handgrip strength in men [−2.4 kg (95% CI − 3.2;−1.5)] and poorer chair rise test in women [−0.8 rises (95% CI − 1.6;−0.1)]. There was no support for interactions. Conclusion: In contrast to earlier studies among older people, no association between loneliness and physical capability was found in this cohort of middle-aged men and women. Loneliness may not yet have resulted in detectable differences in physical capability in this age group. Further research is needed to clarify if, and at what point in the life course loneliness begins to affect physical capability.