We report a biocatalytic platform of engineered cytochrome P450 enzymes to carry out carbene-transfer reactions using a lactone-based carbene precursor. By simply altering the heme-ligating residue, we obtained two enzymes that catalyze olefin cyclopropanation (Ser) or S-H bond insertion (Cys). Both enzymes exhibit high catalytic efficiency and stereoselectivity, thus enabling facile access to structurally diverse spiro[2.4]lactones and α-thio-γ-lactones. Computational studies revealed the mechanism of carbene S-H insertion and explain how the axial ligand controls reactivity and selectivity. This work expands the catalytic repertoire of hemeproteins and offers insights into how these enzymes can be tuned for new chemistry.