In this study, Ag-functionalized porous ZnO nanocomposites were successfully synthesized via pyrolysis of Ag-loaded ZIF-8 precursors. The structural and surface properties of the materials were systematically characterized using XRD, XPS, FESEM, and HRTEM analyses. A gas sensor fabricated from the optimized 3.0 wt% Ag–ZnO sample exhibited a significantly enhanced response (Ra/Rg = 103) toward 100 ppm acetone at an operating temperature of 275 °C, which is approximately 2.51 times greater than that of pristine ZnO. The sensor also demonstrated rapid response/recovery times (6 s/7 s), excellent linearity over a wide concentration range (500 ppb–200 ppm), good selectivity against common interfering VOCs, and stable performance, with over 95% response retention after 30 days. The improved sensing performance is attributed to the hierarchical porous structure derived from ZIF-8 and the increased oxygen vacancy concentration and chemisorbed oxygen species induced by Ag loading, which collectively increase surface reaction activity. This work provides an effective strategy for constructing noble metal-modified porous ZnO materials for sensitive and reliable acetone detection.