Soft electronic fibres are potential building blocks for a variety of emerging technologies including smart textiles and wearable health monitors. However, it remains a challenge to fabricate fibres that combine conductive and dielectric domains in complex architectures in a simple and scalable way. Here we show that a thermal drawing approach can be used to fabricate stretchable fibre-based sensors from liquid-metal-embedded elastomers. The material formulation and processing parameters can be controlled to create high aspect-ratio stretchable fibres that integrate high-conductivity (around 103 S cm−1) and high-dielectric (κ≈13.5) domains across the fibre cross-section. We illustrate the versatility of our approach by creating an all-liquid-metal-based capacitive fibre sensor, which offers a gauge factor of 0.96, stretchability of 925% and high stability to cyclic deformation. We also integrate our fibre-based sensor into textiles and demonstrate an efficient smart knee brace.