Spintronic devices usually rely on long spin relaxation times and/or long spin relaxation lengths for optimum performance. Therefore, the ability to modulate these quantities with an external agent offers unique possibilities. The dominant spin relaxation mechanism in most technologically important semiconductors is the D'yakonov-Perel' (DP) mechanism which may vanish if the spin carriers (electrons) are confined to a single conduction subband in a quantum wire. Here, we report modulating the DP spin relaxation rate (and hence the spin relaxation length) in self assembled 50 nm diameter InSb nanowires with infrared (IR) light at room temperature. In the dark, almost all the electrons in the nanowires are in the lowest conduction subband, resulting in near-complete absence of DP relaxation. This allows observation of spin-sensitive effects in the magnetoresistance. Under IR illumination, higher subbands get populated and the DP spin relaxation mechanism is revived, leading to a three-fold decrease in the spin relaxation length. Consequently, the spin sensitive effects disappear under illumination. This phenomenon may have applications in spintronic room-temperature IR photodetection.