Titanium dioxide (TiO2) photocatalytic powder materials doped with various levels of manganese (Mn) were synthesized to be used as additives to wall painting in combating indoor and outdoor air pollution. The heterogeneous photocatalytic degradation of gaseous acetaldehyde (CH3CHO) on Mn–TiO2 surfaces under ultraviolet and visible (UV/Vis) irradiation was investigated, by employing the Photochemical Static Reactor coupled with Fourier-Transformed Infrared spectroscopy (PSR/FTIR) technique. Experiments were performed by exposing acetaldehyde (~400 Pa) and synthetic air mixtures (~1.01×105 Pa total pressure) on un-doped TiO2 and doped with various levels of Mn (0.1–33% mole percentage) under UV and visible irradiation at room temperature. Photoactivation was initiated using either UV or visible light sources with known emission spectra. Initially, the photo-activity of CH3CHO under the above light sources, and the physical adsorption of CH3CHO on Mn–TiO2 samples in the absence of light were determined prior to the photocatalytic experiments. The photocatalytic loss of CH3CHO on un-doped TiO2 and Mn–TiO2 samples in the absence and presence of UV or visible irradiation was measured over a long time period (≈60 min), to evaluate their relative photocatalytic activity. The gaseous photocatalytic end products were also determined using absorption FTIR spectroscopy. Carbon dioxide (CO2) was identified as the main photocatalysis product. It was found that 0.1% Mn–TiO2 samples resulted in the highest photocatalytic loss of CH3CHO under visible irradiation. This efficiency was drastically diminished at higher levels of Mn doping (1–33%). The CO2 yields were the highest for 0.1% Mn–TiO2 samples under UV irradiation, in agreement with the observed highest CH3CHO decomposition rates. It was demonstrated that low-level (0.1%) doping of TiO2 with Mn results in a significant increase of their photocatalytic activity in the visible range, compared to un-doped TiO2. This elevated activity is lost at high doping levels (1–33%). Finally, the photocatalytic degradation mechanism of CH3CHO on 0.1% Mn–TiO2 surfaces under visible irradiation leading to low CO2 yields is different than that under UV irradiation resulting to high CO2 yields.