Poly(methyl methacrylate) (PMMA) films with 4 wt% of the ionic second order NLO chromophore [(E)-N,N-dimethylamino-N’-methylstilbazolium][p-toluenesulfonate] (DAST), obtained by spincoating of CHCl3–CH3OH (2/1) solutions, show in situ growth of homogeneously dispersed oriented nanocrystals with sizes of less than 100 nm, as supported by TEM and GIXRD evidence. This is achieved by a two step process: corona poling at 8 kV and 90º C followed, in the absence of electric field, by controlled thermal annealing up to 140º C. The final film shows a significant and enduring second harmonic generation (SHG) at room temperature.