A new theoretical model is presented for the mechanical response of a thin-walled bio-mimetic microcapsule upon indentation using an atomic force microscope. The membrane is strained by both bending moments and membrane stress in the presence of a tensile pre-stress. When the intrinsic membrane stress is large compared to the local stress around the indentation dimple, the constitutive relation is linear; and when it is small, a cubic relation is deduced. The new theory is rigorously compared to the existing models of: (i) membrane under pure bending by Boulbitch; and (ii) membrane under pure stretching by Yao et al., which are shown to be two limiting cases of our general solution. Published experimental data in literature show that AFM measurement always involves deformation mode of mixed bending and stretching.