Molecular photoisomerization plays a crucial role in diverse biological and technological contexts. Here, we combine ion mobility spectrometry and laser spectroscopy to characterize the photoisomerization of molecular cations in the gas phase. The target molecular ions, polymethine dye cations 3,3'-diethylthiacarbocyanine (DTC+), are propelled through helium buffer gas by an electric field and are photoisomerized by light from a tunable laser. Photoexcitation over the 450- 570 nm range converts trans-DTC+ to cis-DTC+, noticeably modifying the ions' arrival time distribution. The photoisomerization action spectrum, which has a maximum at 535 nm, resembles the absorption spectrum of DTC+ in solution but is shifted 25 nm to shorter wavelength. Comparisons between measured and calculated mobilities suggest that the photoisomer involves a twist about the second C-C bond in the methine chain (8,9-cis isomer) rather than a twist about the first methine C-C bond (2,8-cis isomer). It is postulated that the excited gas-phase ions internally convert from the S-1 Franck- Condon region to the So manifold and explore the conformational landscape as they cool through He buffer gas collisions. Master equation simulations of the relaxation process in the S-0 manifold suggest that the 8,9-cis isomer is preferred over the 2,8-cis isomer because it lies lower in energy and because it is separated from the trans isomer by a substantially higher barrier. The study demonstrates that the photoisomerization of molecular ions can be probed selectively in the gas phase, providing insights into photoisomerization mechanisms and information on the solvent-free absorption spectrum.