The addition of small amounts of Ag were found to dramatically promote the reaction of Mg and B powder to form MgB₂ during low-temperature synthesis at 550°C and even at 500°C. Co-addition of Ag and SiC enabled C-substitution for MgB₂, even for reactions at 550°C. Although the added Ag formed the Mg-Ag alloy after heat treatment, it was confirmed that Mg-Ag particles mainly existed at the edge of voids without losing their effective current path. In addition, Ag did not dissolve in the MgB₂ phase. These conditions resulted in T_c and J_c performance as high as those where Ag-free MgB₂ was synthesized at high temperatures. Furthermore, Ag-added MgB₂ tapes were successively synthesized with high reproducibility by heating at 500°C for 24 h. Our results will contribute to the development of low-cost MgB₂ materials.