Mechanical energy harvesters are needed for such diverse applications as self-powered wireless sensors, structural and human health monitoring systems, and cheaply harvesting energy from ocean waves. We report nanofiber yarn harvesters that electrochemically convert tensile or torsional mechanical energy into electrical energy. Stretching coiled yarns generated 250 W/kg of peak electrical power when cycled up to 30 Hz, and up to 41.2 J/kg of electrical energy per mechanical cycle, when normalized to the weight of the harvester yarn. Unlike for other harvesters, torsional rotation produces both tensile and torsional energy harvesting and no bias voltage is required, even when electrochemically operating in salt water. Since homochiral and heterochiral coiled harvester yarns provide oppositely directed potential changes when stretched, both contribute to output power in a dual-electrode yarn. These energy harvesters were used in the ocean to harvest wave energy, combined with thermally-driven artificial muscles to convert temperature fluctuations to electrical energy, sewn into textiles for use as self-powered respiration sensors, and used to power a LED and to charge a storage capacitor. The development of “piezoelectrochemical spectroscopy” and insights into the hierarchical origins of capacitance increased fundamental understanding. This work is collaborative with Seon Jeong Kim, Shi Hyeong Kim, Keon Jung Kim, Tae Jin Mun, Changsoon Choi, and Dong Youn Lee of Hanyang University; Carter S. Haines, Na Li, Jiangtao Di, Young Jun Oh, Juan Pablo Oviedo, Shaoli Fang, Kyeongjae Cho, Moon Kim, Enlai Gao, Dawood Albarq, and Xavier Lepró of the University of Texas at Dallas; Julia Bykova and Raquel Ovalle-Robles of Lintec of America; Nan Jiang, Zunfeng Liu, and Run Wang of the Jiangnan Graphene Research Institute; Prashant Kumar, Rui Qiao, and Shashank Priya of Virtinia Tech; and Matthew Steven Lucas, Lawrence F. Drummy, and Benji Maruyama of the Wright-Patterson Air Force Research Laboratory.