The recently reported magnetic phase diagram (T_N versus de Gennes factor deG) of the body-centered tetragonal RCo₂B₂C series indicates that compounds with R=Pr orders at a higher temperature (T_N≈10.2K) than that of the isomorphs R=Nd (T_N=3.0K), Er (T_N=4.0 K), Sm (T_N=6.0K), Ho (T_N=5.4K), Dy (T_N=7.8K) (Rapp, El Massalami, Phys. Rev. B 60 (1999) 3355; El Massalami et al., Phys. Rev. B, submitted). Since the deG of the latter compounds are higher than that of Pr compound, then such non-monotonic evolution of T_N with deG deserves further investigations. In that direction, we synthesized and investigated the magnetic phase diagram of the series Pr_1−xDy_xCo₂B₂C (x=0.0, 0.09, 0.13, 0.345, 0.797, 1.0) wherein deG spans the interval from 0.8 (x=0) to 7.0819 (x=1). The obtained unit-cell parameters are in excellent agreement with the Vegard's law, demonstrating that the Pr and Dy atoms substitute each other randomly. Magnetic measurements reveal that the saturated magnetic moment (at 2K and 90kOe) increases linearly with x (∂μ/∂x≈4.7(5)μ_B). Furthermore, for x<0.17, T_N(x) decreases sharply with x (∂T_N/∂x≈−36(1) K), while for x⩾ 0.17, T_N(x) increases as ∂T_N/∂x≈5.8(1) K. The overall evolution of T_N with deG reproduces satisfactorily the behavior of the above cited RCo₂B₂C compounds. These results will be discussed in terms of the hybridization between the 4f and the conduction electrons.