BACKGROUND AND AIMS: Abdominal aortic calcification (AAC) reflects subclinical atherosclerotic cardiovascular disease (ASCVD), but its lifestyle determinants remain uncertain. We investigated how a Healthy Lifestyle Score (HLS) relates to AAC and whether AAC mediates the HLS-ASCVD association. METHODS AND RESULTS: Participants from the UK Biobank Imaging study were assessed for AAC from DXA images. A Healthy Lifestyle Score (HLS), based on smoking, physical activity, diet quality, alcohol consumption, sleep, and BMI was developed, and categorised as low, moderate and high. Cross-sectional associations with (i) any AAC (AAC≥1) and (ii) high AAC (AAC≥6) were analysed using multivariable-adjusted logistic regression models. The association between HLS and incident ASCVD over 6 years was assessed using Cox regression models. Mediation analysis examined the extent to which the HLS-ASCVD association was mediated by AAC. Among 27,818 participants (52% female; median age 65y), 33.5% had any AAC and 4.4% had high AAC. Compared to participants with a low HLS, those with moderate or high scores had 21% (95%CI: 60%-105%) and 66% (33%-59%) lower odds of having high AAC, respectively, and 10% (78%-104%) and 18% (0.71%-95%) lower odds of having any AAC, respectively. These associations were strongest in women and those younger than 65 years. During follow-up, a one-point increment in the HLS was associated with a 4% (0%-8%) lower rate of incident ASCVD. High AAC accounted for 20% (10%-48%) of this association. CONCLUSION: These findings suggest targeting specific modifiable lifestyle factors, especially earlier in life, may help prevent high AAC levels and subsequent risk of ASCVD.