The first combination of mid-infrared (MIR) tunable quantum cascade lasers (tQCLs) with thin-film diamond strip waveguides (DSWG) suitable for advanced chem/bio sensing is presented. The sensing system comprises thin-film diamond chips fabricated via chemical vapor deposition (CVD) and structured using inductively coupled plasma (ICP) etching serving as photonic waveguide for radiation emitted by a broadly tunable quantum cascade laser (tQCL) in the spectral regime of 5.78-6.35 µm (1570-1730 cm-1). The characterization of such freestanding diamond waveguides reveals excellent transmission properties across a broad MIR band. Exemplarily, the detection of acetone in D2O via evanescent field absorption is demonstrated achieving a limit of detection (LOD) as low as 0.2 nL, which indicates a significant sensitivity improvement compared to conventional MIR slab/strip waveguides reported to date. Providing characteristic absorption features within the tuning range of the tQCL, studies using anisaldehyde as analyte further corroborate the potential of tQCL-DSWG-based chem/bio sensors.