Frequency‐resolved measurements of radiation growth have been performed on a millimeter‐wave free‐electron laser using an intense relativistic electron beam. These measurements have shown large radiation growth rates (approximately 2 dB/cm) over a broad instantaneous bandwidth (66–90 GHz), in good agreement with predictions of theory for operation in the collective regime. Growth narrowing and saturation effects have also been observed. In addition, a large increase in experimental power and efficiency has been observed to result from tapering the strength of the axial magnetic field in the sense that compensates for kinetic energy extraction from the electron beam. Direct calorimetric measurements indicate the production of ≳75 MW centered at 75 GHz with 6% experimental efficiency.