Collinear laser spectroscopy (CLS) has been performed in a multi-reflection time-of-flight (MR-ToF) device operated in single-pass mode, i.e., without confining the ions in the ion trap. While our Multi Ion Reflection Apparatus for Collinear Laser Spectroscopy (MIRACLS) aims to increase the CLS sensitivity by storing ions in the MR-ToF device, the present work characterises conventional single-passage CLS as a preparatory step for the upcoming comparison with MIRACLS’ multi-pass mode. To this end, the isotope shift in the 3s2S1/2→3p2P3/2 transition (D2 line) between ions of the magnesium isotopes 24Mg and 26Mg has been measured under varying experimental conditions. Our result agrees with the precise literature value. Associated studies of systematic uncertainties demonstrate a measurement accuracy of better than 20 MHz in this new apparatus. This value will serve as the reference for analogous studies to be performed in the MIRACLS approach in which ions are trapped in the MR-ToF device for thousands of revolutions and probed by the spectroscopy laser during each passage.