The key requirement for a material to exhibit nonlinear optical (NLO) activity is the presence of non-centrosymmetric (polar) order, an attribute that is usually restricted to certain crystalline classes and ferroelectric liquid crystals1,2. NLO activity can also be obtained in some amorphous organic materials by applying an intense electric field (corona discharge) above the glass transition temperature, T g, and subsequently quenching the field-induced polar orientation order3,4. Such materials are attractive for NLO device applications, as they promise lower costs and easier processibility than their crystalline organic and inorganic counterparts5. But field-induced polar order is not stable, and the eventual return to equilibrium (apolar) order results in a deterioration of NLO activity, particularly at temperatures near T g (refs 6, 7). Here we show that this thermally activated decay of polar order can be circumvented by using a liquid crystal in which both mesogens (molecules that induce a liquid-crystal phase) and NLO-active chromophores are appended to macromolecular siloxane rings. We find that a shear-aligned melt of these composite macromolecules gives rise to a material with a monodomain lamellar superstructure that retains bistable, field-induced polar order above T g. We attribute the thermal stability of these materials to an energetically favoured polar packing arrangement of the constituent macro-molecules.