Background: Targeted cancer therapies almost exclusively disrupt protein function and phosphorylation, but genomics-based biomarkers underpin the vast majority of oncology molecular profiling. The reverse phase protein array (RPPA) technology has been used in the clinical setting to identify potential drug targets and to monitor the effects of treatment on protein expression and phosphorylation. Here, we investigated the Theralink RPPA assay, a highly sensitive CAP/CLIA accredited protein/phosphoprotein profiling technology, across a pan-tumor cohort of 188 samples which had undergone whole exome sequencing and transcriptomics. Methods: Using the Theralink RPPA assay, 32 key protein and phosphoprotein cancer therapy targets were quantified from formalin-fixed, laser capture microdissected (LCM) tumor epithelium across a pan-tumor cohort of 188 cancer cases. All samples had concomitant CLIA-based whole exome and whole transcriptome analysis performed. Most of these tumors were head and neck cancer (34%), uterine cancer (23%), and ovarian/fallopian tube cancer (17%), with the remaining 26% made up of various cancer types, including skin, breast, lung, brain, and other cancers. Results: Of the pan-tumor cohort, 95% of cases had at least one actionable protein target identified, compared to 86% of cases with clinically relevant genomic alterations. Altered genes and Theralink RPPA assay protein targets were broadly classified into 10 biological pathways. The pathway most frequently identified as actionable via Theralink’s RPPA assay was HER-family protein signaling (85% of cases) with concurrent genomic alterations in HER-family genes found for 9% of cases. In contrast, mTOR signaling was found to be the most frequent clinically relevant genomically altered pathway (48% of cases). When using a clinical breast cancer reference for HER2 protein/phosphorylation levels that is guiding HER2-targeted therapy in the unamplified setting, actionable total and phospho-HER2 levels were found in 82% of the pan-tumor cases, overlapping with ERBB2 genomic alterations in 4% of cases. The gene found to be altered the most was TP53 (101 cases; 54%), which was not part of the targeted Theralink RPPA assay. Conclusions: Our data suggest that quantification of key cancer proteins and phosphoproteins using RPPA extend the identification of actionable drug targets beyond what was found with WES/WTS testing alone. Especially the quantification of total and phospho-HER2 identified a significant pan-tumor subpopulation that may benefit from HER2-targeted therapies, including trastuzumab deruxtecan (T-Dxd).