The Cs/Ru/MgO catalyst was synthesized by sequential impregnation of Ru and Cs on MgO support using Ru(NO3)3 and CsNO3 precursors. Catalytic ammonia synthesis was carried out in a fixed-bed flow reactor using H2 and N2 as reactants. The stability of the catalyst was measured at 350 °C, 2.5 MPa gauge pressure, and SV as 1200 h−1 using the H2/N2 ratio 3 as a reactant feedstock. The Cs/Ru/MgO catalyst retained its ammonia synthesis activity while conducting experiments at mild reaction conditions of 325 °C and 350 °C. An increase in experimental temperature to 375−425 °C decreased the ammonia synthesis activity retaining only to 42% of the initial activity after 680 h of time on stream. The deformation of the catalyst’s structure, which was caused by Cs leaching and redistribution of the Ru and increased crystallinity of MgO at high-temperature conditions, was considered the plausible reason for the drastic decrease in ammonia synthesis activity.