This article presents the effects of an imidazolium-based ionic liquid (IL) on the thermodynamics and in-plane viscoelastic properties of model membranes of anionic phospholipids. The negative Zeta potential of multilamellar vesicles of 14 carbon lipid 1,2-dimyristoyl-sn-glycero-3-phospho-(1'-rac-glycerol) (DMPG) is observed to reduce due to the presence of few mole % of an IL 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM][BF4]). The effect was found to be stronger on enhancing the chain length of the lipid. The surface pressure–area isotherms of lipid monolayer formed at air–water interface are modified by the IL reducing the effective area per molecule. Further, the equilibrium elasticity of the film is altered depending upon the thermodynamic phase of the lipids. While the presence of the IL in the DMPG lipid makes it ordered in the gel phase by reducing the entropy, the effect is opposite in the fluid phase. The in-plane viscoelastic parameters of the lipid film is quantified by dilation rheology using the oscillatory barriers of a Langmuir trough. Even though the low chain lipid DMPG does not show any effect of IL on its storage and loss moduli, the longer chain lipids exhibit a prominent effect in the liquid extended (LE) phase. Further, the dynamic response of the lipid film is found to be distinctly different in the liquid condensed (LC) phase from that of the LE phase. © 2022, The Author(s), under exclusive licence to Springer Science+Business Media, LLC, part of Springer Nature.