[en] Highlights: • A novel liquid metal solvent based co-segregation (LMSCS) strategy was employed to directly fabricate 2D WC single crystals embedded in graphene film, i.e. a unique in-plane heterostructure, over a large scale in one step via chemical vapor deposition (CVD). • This 2D in-plane WC–graphene heterostructure (i-WC–G) was firstly applied in HER. Basing on the high crystallinity of the WC and utilizing the interfacial synergistic catalytic effects, the 2D i-WC–G heterostructure exhibited excellent electrocatalytic activity for the HER. • The overpotential was as low as 120 mV and the Tafel slope was 38 mV/decade, which indicated higher performance and efficiency than mainstream 2D HER catalysts. • The reproducible polarization plot even after 5000 voltammetry (CV) cycles highlighted the excellent durability of our developed 2D i-WC–G heterostructure. • Such a versatile synthesis approach will allow the fabrication of other high-quality 2D transition metal carbides (TMCs) and their embedding in in-plane structures and this will promote the practical catalytic application of metal carbides. Electrochemical water splitting is regarded as one of the most economical and eco-friendly approaches for hydrogen revolution. Developing a low-cost and earth-abundant non-noble-metal catalyst will be of the most significance. Tungsten carbide (WC) is highly promising due to its platinum (Pt) -like behavior in surface catalysis. Here we first report a liquid metal solvent based co-segregation (LMSCS) strategy to fabricate a high uniformity of 2D WC crystals embedded in graphene by chemical vapor deposition (CVD) in one step. The 2D in-plane WC–graphene heterostructures (i-WC–G) are remarkably stable under an electro-catalytic environment and ensure good interfacial synergy between the 2D WC crystallites and graphene to achieve a more effective hydrogen evolution. The overpotential is as low as 120 mV and the Tafel slope is 38 mV/dec, which indeed exhibits outstanding catalytic potential among the reported 2D material systems. Our elegant and versatile approach allows the fabrication of other high-quality 2D transition metal carbides (TMCs) and their in-plane heterostructures, which will further promote practical catalytic applications of metal carbides.