Semisynthetic proteins engineered to contain non-coded elements such as post-translational modifications (PTMs) represent a powerful class of tools for interrogating biological processes. Here, we introduce a one-pot, chemoenzymatic method that allows broad access to chemically modified proteins. The approach involves a tandem transamidation reaction cascade that integrates intein-mediated protein splicing with enzyme-mediated peptide ligation. We show that this approach can be used to introduce PTMs and biochemical probes into a range of proteins including Cas9 nuclease and the transcriptional regulator MeCP2, which causes Rett syndrome when mutated. The versatility of the approach is further illustrated through the chemical tailoring of histone proteins within a native chromatin setting. We expect our approach will extend the scope of semisynthesis in protein engineering.