A novel boron-modified polysilazane functionalised with Si- and N-bonded methyl groups has been synthesised and characterised by means of FT-IR and NMR spectroscopies as well as elemental analysis. Both Si- and N-bonded methyl groups inhibited extensive cross-linking to yield a tractable polymer which was successfully processed into polymer green fibers by a melt-spinning process. After the shaping processing, the N-bonded methyl groups offered the capability of facile polymer cross-linking in an ammonia atmosphere at 200 °C to increase the ceramic yield of the polymer and avoid inter-fiber fusion. The as-obtained cured fibers were subsequently pyrolysed at 1400 °C in a nitrogen atmosphere to provide amorphous and stable Si3.0B1.0C5.0N2.4 ceramic fibers with ca. 1.3 GPa in tensile strength, ca. 170 GPa in Young's modulus and ca. 23 μm in diameter.