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Phosphorylation is an important post‐translational modification on proteins involved in many cellular processes; however, understanding of the regulation and mechanisms of global phosphorylation remains limited. Herein, we utilize self‐assembled monolayers on gold for matrix‐assisted laser desorption/ionization mass spectrometry (SAMDI‐MS) with three phosphorylated peptide arrays to profile global phosphatase activity in cell lysates derived from five mammalian cell lines. Our results reveal significant differences in the activities of protein phosphatases on phospho‐ serine, threonine, and tyrosine substrates and suggest that phosphatases play a much larger role in the regulation of global phosphorylation on proteins than previously understood. [ABSTRACT FROM AUTHOR]