A micro-mechanical finite element model of a composite sublaminate is proposed to study the mechanical response of ultra-thin plies, consisting of a representative volume element of a 90° thin lamina in-between two homogenised±θ° plies. Random fibre distributions, materially and statistically equivalent to real distributions, are analysed. A three-dimensional computational micro-mechanics framework, with a special focus on the elastic–plastic and damage constitutive behaviours of the matrix and on the response of the fibre–matrix interface, is used in the present analysis. Varying the 90° ply thickness, it is possible to assess its effect on the mechanical response of laminated composites—the in situ effect. The proposed framework is able to accurately represent the micro-mechanical response of ultra-low grades, including (i) the mechanics of transverse cracking onset and propagation,(ii) the constraining effect observed in the laminae embedded in multidirectional laminates,(iii) the gradual, slow stress relaxation and progressive transverse cracking observed in very thin plies, and consequent increase of the crack density,(iv) the reduction in crack-opening displacement of the transverse cracks with ply thinness,(v) the formation of thin necks of matrix material around the regions where interfacial damage is more pronounced, and (vi) the in situ strengths.