Np and Pu are two important actinides of concern for the safe long-term disposal of nuclear waste. Both actinides are, in addition, constituents of global nuclear fallout. Investigation of their environmental behavior requires ultra-sensitive analytical methods, but current methods for a concurrent determination in clay minerals are lacking. In the present study, a Pu isotope was investigated for use as a non-isotopic yield tracer for Np in extraction, purification, and mass spectrometric determination of Np and Pu isotopes in clay materials. Inductively coupled plasma mass spectrometry was used in this developmental study, but the method is intended for future ultra-trace analysis of global-fallout Np and Pu in clay-rich soil materials by the more sensitive accelerator mass spectrometry. Another field of application may be the investigation of diffusion patterns of actinides in compacted clay liners and potential host rocks for radioactive waste disposal. The analytical procedure includes the following steps: (1) extraction of Np and Pu from clay samples; (2) adjustment of Np and Pu to Np(IV) and Pu(III); (3) pre-concentration of Np and Pu by co-precipitation with iron hydroxide; (4) adjustment of Pu to Pu(IV); (5) extraction chromatographic separation of Pu and Np from iron and matrix elements; and (6) determination of Np and Pu by mass spectrometry. The analytical procedure was applied successfully to spiked montmorillonite and illite test portions of up to 1 g. High chemical yields near 90% were obtained for both Np and Pu. The suitability of Pu as a non-isotopic tracer for Np was indicated by Np/Pu chemical yield ratios close to unity. Accurate pH adjustment during the reductive co-precipitation and short processing times are vital to obtain high chemical yields and Np/Pu yield ratios close to unity.