In the present study, polycaprolactone/polyethylene glycol (PCL/PEG) electrospun nanofibres with different anthocyanin (1%, 2%, 3%, and 5%) were fabricated for the instant measurement of pH, especially for applications—such as food freshness detection—where quick response is required. The solution, surface, chemical, thermal, wettability, mechanical, and release properties of the samples were evaluated by viscosity measurements, scanning electron microscopy (SEM), Fourier-transform infrared (FTIR), contact angle measurements, and tensile tests, respectively. The colorimetric analyses were also investigated against the solutions at different pH values and bacterial solutions. Finally, the on-site performance of the sensor was evaluated. Anthocyanin addition initially lowered the solution viscosity, resulting in thinner fibres with a diameter of 288 nm. The diameters were increased up to 395 nm with the increasing anthocyanin. Anthocyanin addition enhanced the wettability and the mechanical properties, and the contact angles decreased to 43°. The highest modulus was observed for 1% anthocyanin, with a value of 6.162. The release experiments revealed that the anthocyanin-loaded samples released a large amount of anthocyanin (between ~12% and 38%) in the first 15 s. The colorimetric analyses showed that PCL/PEG nanofibre mats with 2% and 3% anthocyanin concentrations were the most capable pH-sensitive sensors for detecting pH changes from 2 to 8. As a result, it can be concluded that 3% anthocyanin is the threshold value for the production of the anthocyanin-loaded nanofibre mats, and these structures are promising for the instant detection of pH proved by the on-site application.