Patients with chronic liver disease are systematically excluded from acute myocardial infarction (AMI) trials, and prior real-world data rely on administrative coding alone. Whether ICD-coded liver disease and laboratory-defined liver fibrosis identify the same patients, and whether they predict the same outcomes, is unknown. We conducted a single-center retrospective cohort study of 1037 consecutive adults admitted with AMI (ICD-10 I21.x) to a tertiary New York center between November 2022 and December 2024. The primary exposure was ICD-defined advanced liver disease (cirrhosis, hepatic failure, or portal hypertension/decompensation; n = 102). The secondary, lab-based exposure was the Fibrosis-4 (FIB-4) index calculated from earliest admission AST, ALT, and platelet count (computable in 1031 patients, 99.4%), stratified as low (3.25). Co-primary outcomes were invasive management (diagnostic angiography, percutaneous coronary intervention, or coronary artery bypass grafting) and in-hospital mortality. Multivariable logistic regression adjusted for age, sex, diabetes, chronic kidney disease, heart failure, and ST-elevation; the trend across FIB-4 tiers was assessed with the Cochran–Armitage test. Denominators throughout (including the 168/909 occult-fibrosis estimate) use the full exposure group as denominator under a missing-as-not-exposed convention; the four no-LD and two advanced-LD patients with missing FIB-4 are counted as non-advanced fibrosis for this calculation. Patients with ICD-defined advanced liver disease received invasive management less often (12.7% vs. 50.4%; adjusted odds ratio [aOR] 0.17, 95% CI 0.09–0.32) and died in hospital more often (43.1% vs. 7.9%; aOR 8.22, 95% CI 5.02–13.46) than patients without coded liver disease. Outcomes worsened monotonically across FIB-4 tiers (mortality 4.4% → 9.2% → 27.5%; invasive management 55.2% → 45.6% → 33.5%; both p < 0.001 by Cochran–Armitage trend test). Critically, 168 of 909 patients with no coded liver disease (18.5%) had FIB-4 > 3.25, representing a substantial population of unrecognized advanced fibrosis missed by clinical coding. Coded liver disease identifies a small, severely affected subgroup with markedly lower rates of invasive management and 6- to 8-fold higher mortality after AMI. Routine FIB-4 calculation, a free, three-variable lab score, identifies a much larger population with occult advanced fibrosis and graded excess risk that ICD codes miss entirely. Pending prospective validation, FIB-4 may serve as a low-cost adjunct to bedside risk stratification in AMI care.