The use of superconducting microresonators together with microwave parametric amplifiers that operate at the quantum noise limit has enhanced the sensitivity of pulsed electron spin resonance (ESR) measurements by more than four orders of magnitude [1]. So far, the microwave resonators and quantum amplifiers have been designed as separate components due to the incompatibility of conventional superconducting quantum-limited amplifiers with even moderate magnetic fields. This has produced complex spectrometers and raised technical barriers toward adoption of the technique. Here, we circumvent this issue by coupling an ensemble of spins directly to a magnetic field–resilient superconducting microwave resonator that exhibits a weak nonlinearity in the form of kinetic inductance [2]. We perform pulsed ESR measurements with a 1-pL mode volume containing 6×10^7 spins and use the weak nonlinearity to amplify the resulting echo signals within the same device. When considering only those spins that contribute to the detected signals, we find a sensitivity of 2.8×10^3 spins/Hz^0.5 for a Hahn echo sequence at a temperature of 400 mK [3]. In situ amplification is demonstrated at fields up to 254 mT, highlighting the technique’s potential for application under conventional ESR operating conditions.