Aims: This study aimed to test the hypotheses that a high total sitting time and vigorous physical activity in leisure time increase the risk of low back pain and herniated lumbar disc disease. Methods: A total of 76,438 adults answered questions regarding their total sitting time and physical activity during leisure time in the Danish Health Examination Survey 2007–2008. Information on low back pain diagnoses up to 10 September 2015 was obtained from The National Patient Register. The mean follow-up time was 7.4 years. Data were analysed using Cox regression analysis with adjustment for potential confounders. Multiple imputations were performed for missing values. Results: During the follow-up period, 1796 individuals were diagnosed with low back pain, of whom 479 were diagnosed with herniated lumbar disc disease. Total sitting time was not associated with low back pain or herniated lumbar disc disease. However, moderate or vigorous physical activity, as compared to light physical activity, was associated with increased risk of low back pain (HR = 1.16, 95% CI: 1.03–1.30 and HR = 1.45, 95% CI: 1.15–1.83). Moderate, but not vigorous physical activity was associated with increased risk of herniated lumbar disc disease. Conclusions: The results suggest that total sitting time is not associated with low back pain, but moderate and vigorous physical activity is associated with increased risk of low back pain compared with light physical activity.