Introduction: Mortality is an important long-term indicator of the public health impact of chronic kidney disease (CKD). We investigated the role of individual comorbidities and multimorbidity on age-specific mortality risk among US veterans with new-onset CKD. Methods: The cohort included 892,005 veterans aged ≥18 years with incident CKD stage 3 between January 2004 and April 2018 in the US Veterans Health Administration (VHA) system and followed until death, December 2018, or up to 10 years. Incident CKD was defined as the first-time estimated glomerular filtration rate (eGFR) was <60 mL/min/1.73 m2 for >3 months. Comorbidities were ascertained using inpatient and outpatient clinical records in the VHA system and Medicare claims. We estimated death rates for any cardiovascular disease (CVD, a composite of 6 CVD conditions) and 15 non-CVD comorbidities, and adjusted risks of death (hazard ratio [HR], 95% confidence interval [CI]) overall and by age group at CKD incidence. Results: At CKD incidence, the mean age was 72 years, and 97% were male; the mean eGFR was 52 mL/min/1.73 m2, and 95% had ≥2 comorbidities (median, 4) in addition to CKD. During a median follow-up of 4.5 years, among the 16 comorbidities, CVD was associated with the highest relative risk of death in younger veterans (HR 1.96 [95% CI: 1.61-2.37] in ages 18-44 years and HR 1.66 [1.63-1.70] in ages 45-64 years). Dementia was associated with the highest relative risk of death among older veterans (HR 1.71 [1.68-1.74] in ages 65-84 years and HR 1.69 [1.65-1.73] in ages 85-100 years). The additive effect of multimorbidity on risk of death was stronger in younger than older veterans. Compared to having 1 or no comorbidity at CKD onset, the risk of death with ≥5 comorbidities was >7-fold higher among veterans aged 18-44 years and >2-fold higher among veterans aged 85-100 years. Conclusion: The large burden of comorbidities in US veterans with newly identified CKD places them at the risk of premature death. Compared with older veterans, younger veterans with multiple comorbidities, particularly with CVD, at CKD onset are at an even higher relative risk of death.