  The use of superconducting micro-resonators in combination with
quantum-limited Josephson parametric amplifiers has in recent years lead to
more than four orders of magnitude improvement in the sensitivity of pulsed
Electron Spin Resonance (ESR) measurements. So far, the microwave resonators
and amplifiers have been designed as separate components, largely due to the
incompatibility of Josephson junction-based devices with even moderate magnetic
fields. This has led to complex spectrometers that operate under strict
environments, creating technical barriers for the widespread adoption of the
technique. Here we circumvent this issue by inductively coupling an ensemble of
spins directly to a weakly nonlinear microwave resonator, which is engineered
from a magnetic field-resilient thin superconducting film. We perform pulsed
ESR measurements with a $1$~pL effective mode volume and amplify the resulting
spin signal using the same device, ultimately achieving a sensitivity of $2.8
\times 10^3$ spins in a single-shot Hahn echo measurement at a temperature of
400 mK. We demonstrate the combined functionalities at fields as large as
254~mT, highlighting the technique's potential for application under more
conventional ESR operating conditions.
