The light weight and complementary properties of Mg and Ti give Mg/Ti composites great development potential. The design and exploitation of high-quality and low-cost fabrication technologies support the advancement of Mg/Ti composites. In this study, Mg/Ti composites were fabricated without interlayer by reaction-diffusion between Al and Ti in AZ91 and TA2 alloys, respectively. The formation order of the interfacial intermetallic was deduced by thermodynamics calculations. The feasibility of interlayer-free diffusion bonding Mg/Ti composites held for 1–3 h was scrutinized by observing the microstructural evolution of the Mg/Ti interface and testing the mechanical properties of the Mg/Ti composites. The results indicated that with increasing holding time, diffusion bonding became more compact and joint quality improved. A high-quality bond formed between the Mg and Ti held for 3 h. The interfacial intermetallic layer formed a continuous and compact Al3Ti + Ti3Al duplex structure. The contents of the Al3Mn nanophase and Mg17Al12 decreased gradually with increasing holding time. Owing to the highest volume of Mg attached to the Ti side and the formation of a few dimples, the tensile strength and elongation of the Mg/Ti composite were increased by 88.8 and 99.6%, respectively, by extending the holding time to 3 h. •Al preferentially reacted with Ti below 427 °C, which provided concept for bonding insoluble and nonreactive Mg/Ti.•Interlayer-free Mg/Ti composites were fabricated by diffusion bonding successfully.•Interfacial intermetallic layer formed continuous, compact Al3Ti/Ti3Al duplex structure .•At 3 h, tensile strength and elongation increased by 88.8 and 99.6%, respectively.