Four isoprenylated flavonols, including two new compounds, macainermisins A-B (1-2), and two known compounds, sinoflavonoid P (3), broussoflavonol F (4), were isolated from the leaves of Macaranga inermis. A combination of HRESIMS, UV, 1D, and 2D NMR spectra elucidated the structures of 1-2. Flavonols (1-4) were evaluated against three cancer cells. Compound 1 showed high cytotoxicity against WiDR with an IC50 value of 0.93 µM, and compound 2 was active towards HeLa and WiDR (IC50 values of 0.90 and 0.94 µM), and compound 3 showed high activity towards 4T1 and HeLa (IC50 values of 0.83 and 0.98 µM).