It is estimated that 10% of individuals with a myeloid malignancy carry a germline susceptibility. Using the genome-first approach, in which individuals were ascertained on genotype alone, rather than clinical phenotype, we quantified the prevalence and penetrance of pathogenic germline variants in eight myeloid malignancy predisposition (gMMP) genes. ANKRD26, CEBPA, DDX41, MECOM, SRP72, ETV6, RUNX1 and GATA2, were analyzed from the Geisinger MyCode DiscovEHR (n = 170,503) and the United Kingdom Biobank (UKBB, n = 469,595). We identified a high risk of myeloid malignancies (MM) (odds ratio[OR] all genes: DiscovEHR, 4.6 [95% confidential interval (CI) 2.1-9.7], p < 0.0001; UKBB, 6.0 [95% CI 4.3-8.2], p = 3.1 × 10-27), and decreased overall survival (hazard ratio [HR] DiscovEHR, 1.8 [95% CI 1.3-2.6], p = 0.00049; UKBB, 1.4 [95% CI 1.2-1.8], p = 8.4 × 10-5) amongst heterozygotes. Pathogenic DDX41 variants were the most commonly identified, and in UKBB showed a significantly increased risk of MM (OR 5.7 [95% CI 3.9-8.3], p = 6.0 × 10-20) and increased all-cause mortality (HR 1.35 [95% CI 1.1-1.7], p = 0.0063). Through a genome-first approach, this study genetically ascertained individuals with a gMMP and determined their MM risk and survival.