The modular cosmetic pen is an apparatus that compacts a writing instrument and a cosmetic tool together while preserving a streamline and ergonomic structure. The apparatus includes a writing instrument, an adapter, at least one cosmetic tool, and a cover. The writing instrument is preferably a pen; however, the writing instrument may be a variety of applied writing instruments. The writing instrument preferably includes a twist mechanism that extends and retracts a dispensing tip. The dispensing tip preferably expels ink. The adapter connects the at least one cosmetic tool to the writing instrument. The at least one cosmetic tool is connected to the adapter, positioned opposite the writing instrument. The at least one cosmetic tool includes a cosmetic applicator and a cap, and preferably a tube which serves as a reservoir for a cosmetic. The cover is removably attached to the adapter over the at least one cosmetic tool.