In the present work, imidazolinium L-tartrate (IMLT) crystals and L-proline-doped IMLT crystals were grown by slow cooling solution growth technique. The powder X-ray diffraction technique reveals the lattice parameters and strains developed due to the doping of L-proline in 1, 3 and 5 mol concentrations. The transmittance spectrum unveils the contribution of the dopant atoms on the optical property of the grown crystals. Bandgap and cut-off wavelength are affected by the increase in the dopant concentration. The vibrational frequencies around 3132, 2925 and 1438 cm−1 are due to the O–H stretching in the carboxyl group, C–H stretching vibration of carboxyl group and aromatic ring vibrations, respectively, present in the grown crystals are observed in the FTIR spectroscopic analysis. The dielectric constant and dielectric loss of pure and L-proline-doped IMLT crystals with respect to the frequency of the applied electric field have been investigated by the dielectric measurements at room temperature. The surface morphology of the grown crystals was examined by the etching study which reveals the growth mechanism of pure and L-proline-doped IMLT crystals. Etch pits in rectangular pattern were found to appear. The percentages of carbon, hydrogen and nitrogen elements present in pure and L-proline-doped IMLT crystals were determined using CHN analysis. The third-order nonlinear susceptibility was calculated from the nonlinear absorption coefficient and nonlinear refractive index data obtained from the Z-scan analysis. The dopant, L-proline, has altered the properties of IMLT single crystal. © 2021, The Author(s), under exclusive licence to Springer Science+Business Media, LLC part of Springer Nature.