In this paper, a matrix-free strategy based on the analysis of nitrocellulose membranes (NCMs) modified with gold nanoparticles (AuNPs) is described, using pulsed-laser desorption ionization mass spectrometry (LDI-MS) for comprehensive quantification of lead ions (Pb) with a sub-nanomolar sensitivity in complicated biofluids. The strong hydrophobic interactions between the NCM and bovine serum albumin (BSA) lead to trapping of BSA-modified AuNPs (BSA-AuNPs), resulting in the formation of a nanocomposite film of BSA-AuNPs on the membrane (BSA-AuNP/NCM). When the AuNPs interact with thiosulfate (S2O32−) ions in solution, Au+·S2O32− complexes form on the AuNP surfaces, facilitating the deposition of Pb atoms in the form of PbAu alloys in the presence of Pb2+ ions. The BSA-AuNP/NCM nanocomposite is a useful LDI-MS matrix because it allows: i) the soft and enhanced ionization of Pb−Au alloys from the AuNP surfaces; ii) accurate mass measurements (precision: 5 ppm) of Au, Pb, and Au–Pb ions; iii) the extraction of Pb2+ ions from very-dilute aqueous solutions (1.0 × 10−9M); and iv) analyses to be performed directly after the introduction of the substrate into the mass-analysis LDI spectrometer (i.e., without the need for an elution process). In contrast to the noisy spectra typically obtained when using other AuNP-assisted LDI approaches, our homogeneous BSA-AuNP/NCM nanocomposite provides clean mass spectra with fewer and weaker signals from AuNP-associated interfering species. As a result, the BSA-AuNP/NCM substrates allow sensitive LDI-MS detection of analytes with low mass-to-charge ratios. Under optimal conditions, this LDI-MS approach provides high sensitivity, a wide dynamic detection range (1.0 × 10−9–5.0 × 10−6M), and a high selectivity toward Pb2+ ions (with at least a 100-fold concentration tolerance relative to other metal ions). The BSA-AuNP/NCM nanocomposite also provides excellent shot-to-shot (<5%) and sample-to-sample (<5%) reproducibilities of ion production because of its homogeneous substrate surface, thereby enabling LDI-based measurements to a consistent quantification of Pb2+ ions in real samples (e.g., urine, whole blood).