A novel, facile, and versatile approach for preparing highly durable, electrically conductive cotton yarns is reported. Polyelectrolyte brushes, a polymer that covalently tethers one end on a surface, are first grown from cotton surfaces by surface-initiated atomic transfer radical polymerization. Subsequent electroless deposition of metal particles onto the brush-modified cotton yarns yields electrically conductive yarns, which have conductivity as high as approximately 1 S/cm and can be used as electrical wires in wearable, flexible electronic devices. Importantly, the formation of polymer brush-bridged metal/cotton hierarchical structures provides robust mechanical and electrical durability to the yarns under many stretching, bending, rubbing, and washing cycles. With proper selection of metal, the conductivity of the samples remains stable after they are stored in air for a few months. This chemical approach can be extended as a general method for making conductive yarns and fabrics from all kinds of natural fibers.