Methods are provided for detecting and quantifying the Mesothelin protein (MSLN) in biological samples that have been fixed in formalin using Selected Reaction Monitoring (SRM)/Multiple Reaction Monitoring (MRM) mass spectrometry. The biological sample may be, for example, tissues and cells treated with formaldehyde containing agents/fixatives including formalin-fixed tissue/cells, formalin-fixed/paraffin embedded (FFPE) tissue/cells, FFPE tissue blocks and cells from those blocks, and tissue culture cells that have been formalin fixed and or paraffin embedded. A protein sample is prepared from the biological sample and the MSLN protein is quantitated by SRM/MRM mass spectrometry by quantitating one or more MSLN fragment peptides in the protein sample.