Aim: The mechanisms behind the association between adult offspring's socioeconomic position and their parents' mortality are not well understood. This study investigates lifestyle-related diseases as a potential mediating pathway between adult offspring's education and parental mortality. Methods: This nationwide register-based cohort study consists of 963,742 older adults aged 65 years between 2000 and 2018. Lifestyle-related diseases were measured between 60 and 65 years and those with prior lifestyle-related diseases were excluded. Natural Effect Models were performed to assess potential mediation through lifestyle-related diseases of the association between offspring's education and parental mortality measured by additive hazard estimates with 95% confidence intervals (CIs). Results: Between 60 and 65 years, 150,501 (15.6%) older adults were diagnosed with lifestyle-related diseases and 149,647 (15.5%) died during follow-up. Compared with having offspring with long education, short education was associated with 631 (95% CI: 555; 707) and 581 (95% CI: 525; 638) additional deaths per 100,000 person-years for women and men, respectively, of which 15.4% (95% CI: 9.0; 21.6) and 16.8% (95% CI: 14.6; 18.9) were mediated by lifestyle-related diseases. The corresponding numbers for medium education were 276 (95% CI: 205; 347) and 299 (95% CI: 255; 343) with 26.2% (95% CI: 12.0; 40.6) and 27.6% (95% CI: 25.1; 31.8) mediated by lifestyle-related diseases. Conclusions: Lifestyle-related diseases accounted for 15-28% of the association between offspring's education and parental mortality for both men and women.