We report a dual-parameter optical fiber sensor based on a line-by-line femtosecond laser–inscribed long-period fiber grating (LPFG) in an exposed-core microstructured fiber (ECF). Owing to the large effective refractive index separation between guided mode groups in the ECF, stable and well-defined coupling from the fundamental mode to selected higher-order core modes is achieved, yielding resonance depths exceeding 15 dB. By integrating the LPFG with a Mach–Zehnder interferometer formed by intermodal interference in an SMF–ECF–SMF configuration, the LPFG provides the dominant sensing response, while the MZI serves as an independent reference signal for mitigating the cross-sensitivity between refractive index and temperature, enabling simultaneous dual-parameter measurement. The proposed platform exhibits refractive-index and temperature sensitivities of 1,675 nm/RIU and −147.2 pm/°C, respectively, representing a substantial improvement over a comparable single-mode fiber device. Importantly, the device requires no tapering, etching, or post-processing, preserving the original fiber geometry and providing improved mechanical robustness compared with tapered or etched fiber sensors, while maintaining stable spectral evolution and straightforward system integration. This work demonstrates a practical route to stable LPFG operation in multimode fibers and establishes a compact, fabrication-efficient platform for high-resolution biochemical sensing and precision measurements in complex environments.