Introduction: Age-related gait speed declines are associated with an increased risk of falls and a loss of autonomy in older adults. Understanding the biological pathways and identifying the biomarkers associated with this decline could allow better and earlier detection of older adults at risk of mobility decline. The objective was to determine whether the age-related decline in gait speed was mediated by biomarkers from blood and urine samples known to be associated with metabolic and hormonal factors, inflammation and immune function, renal function, lipid profile, bone and collagen markers, vitamins, antioxidants, and stress. Methods: Parcipants (n=1030) from the Midlife in the United States (MIDUS) study, aged between 34 and 84 years (55% women), were included in the mediation analyses with Age as a predictor, the average time of two 50-foot-walk trials as an outcome, and 54 biomarkers as mediators. Results: Blood inflammation markers (IL6 and IL8) were the strongest mediators of age-related gait speed declines (22% and 15% of the total effect, respectively), followed by metabolic markers (blood hemoglobin, IGF-1, blood fasting glucose, corresponding to 10%; 7%, 4% of the total effect, respectively). Conclusion: Declines in gait speed may be caused by changes in blood biomarkers, particularly inflammation and metabolic markers, emphasizing their crucial role in understanding mobility decline. These findings could help early detection and intervention.