The excited-state relaxation dynamics of D149, one of the metal-free substituted indoline dyes used in dye-sensitized solar cells, is studied in the whole composition range of the 1-butyl-3-methylimidazolium tetrafluoroborate (BmimBF4)–acetonitrile binary mixture by using time-integrated absorption, emission, and time-resolved transient absorption (TA) spectroscopies. The comparative analysis of absorption and emission spectra indicates that the value of Stokes shift reduces monotonically with decreasing mixture polarity. The global analysis of time-resolved TA spectra indicates the presence of four different time components related to different processes in the excited state of the dye. Importantly, the observed timescales are highly sensitive to composition, polarity, and viscosity of the binary mixture. Increase of viscosity and decrease of polarity observed for increasing ionic liquid (IL) content in the mixture lead to overall increase in the emission lifetime (S1–S0) of D149. At a lower IL mole fraction (XIL = 0.1), the emission lifetime shows a minimum that can be traced back to the change from the situation in which the local environment of the dye is dominated by the interactions in acetonitrile to that in which it is dominated by those in BmimBF4. This also is reflected in the occurrence of a minimum in relative quantum yield in the same range of XIL. The origin of the other moderately long-time component (33 ps in ACN-120 ps in BmimBF4) is still debatable; however, for pure IL and all the mixtures, the composition dependence of this timescale is similar to that of the longest emission lifetime.