Flax stems are often considered waste material. However, since flax fibre has superior mechanical properties amongst natural fibres, it can be used as reinforcement in cementitious composites. Durability of flax, however, is endangered in alkaline environments by the deterioration of alkali-sensitive pectin and hemicellulose. Cottonisation of flax not only makes flax suitable for producing textiles; it divides the technical fibre into bundles of elementary fibres and partially removes the alkali-sensitive pectin and hemicellulose. Therefore, the characteristics of cottonised flax (CF) in fibre reinforced cementitious composites were assessed. The fibre and composite properties were compared with technical flax (TF) fibres and synthetic polyvinyl alcohol fibres. Strength/strain curves of the natural fibre reinforced cementitious materials were analysed to evaluate the mechanical performance. Not only were tensile strength and stiffness evaluated, but also first crack strength, peak stress, work of fracture as a measure for multiple cracking (MC), and the visual closure of individual cracks by self-healing. Cottonisation of flax enhanced the modulus of elasticity, the peak stress and the strength at first crack formation of cementitious materials, in comparison to TF fibres. The self-healing of cracks was independent of the fibre type. Cracks narrower than 30 μm healed completely and crack widths between 30 μm and 150 μm only partly healed.