This research focuses on developing architected lightweight cementitious cellular composites (LCCCs) for potential thermal insulation purposes. Specifically, Voronoi cellular structures with different randomness are adopted and used as cellular structure of the LCCCs. the mechanical properties of the developed LCCCs are investigated by experimental and numerical methods. With the help of 3D printing technique, LCCCs are prepared with two mixtures: a reference mortar, denoted as REF and mortar incorporating microencapsulated phase change materials (mPCMs), denoted as PCM14. Mechanical properties of developed structures are first investigated. The compressive behaviour of the LCCCs is investigated at the age of 28 days. In general, the LCCCs show anisotropic compressive behaviour in two loading directions: the compressive strength and stiffness of the LCCCs in the out-of-plane direction is substantially higher than that of the in-plane direction. Comparing to conventional foam concrete with the same density, the out-of-plane compressive strength of the LCCCs is higher than commonly reported cases in literature. Due to the high porosity of the cellular structures, the compressive strength of the LCCCs is significantly reduced comparing to the corresponding constituent materials, as expected. In addition, this strength reduction of the LCCCs made with PCM14 is substantially lower than the that of the REF samples. Interestingly, the relative stiffness (measured stiffness of the LCCCs normalized by their porosity) of the mPCMs incorporated LCCCs is even higher than the constituent material E-modulus. Furthermore, the critical role of air voids on the compressive behaviour of the LCCCs is identified. The thermal conductivity of the LCCCs is investigated by numerical models using commercial software Abaqus and ANSYS. It is found that the thermal conductivity of the LCCCs is comparable with the conventional foam concrete of the same density. The geometry heterogeneity of the LCCCs has a minor effect on determining the thermal conductivity of the LCCCs. Moreover, increasing porosity or reducing thermal bridge on the heat transfer direction can dramatically improve the thermal insulation performance of the LCCCs. It is found that the natural convection of the air within the cellular pores and the radiation has less notable effect on the thermal conductivity of specimen. Comparing to the REF, PCM14 has enhanced thermal insulation performance. In the end, using extrusion-based 3D concrete printing technique, the LCCCs with mPCMs has been successfully printed as an implement of manufacturing LCCCs as a construction-scale brick. In general, the developed LCCCs have high porosity gives good insulation properties as well as good compressive strength. This gives them great potential to be used as novel lightweight thermal insulation materials for construction.