Device applications of graphene such as ultrafast transistors and photodetectors benefit from the combination of both high-quality p- and n-doped components prepared in a large-scale manner with spatial control and seamless connection. Here we develop a well-controlled chemical vapour deposition process for direct growth of mosaic graphene. Mosaic graphene is produced in large-area monolayers with spatially modulated, stable and uniform doping, and shows considerably high room temperature carrier mobility of ~5,000 cm(2) V(-1) s(-1) in intrinsic portion and ~2,500 cm(2) V(-1) s(-1) in nitrogen-doped portion. The unchanged crystalline registry during modulation doping indicates the single-crystalline nature of p-n junctions. Efficient hot carrier-assisted photocurrent was generated by laser excitation at the junction under ambient conditions. This study provides a facile avenue for large-scale synthesis of single-crystalline graphene p-n junctions, allowing for batch fabrication and integration of high-efficiency optoelectronic and electronic devices within the atomically thin film.