This paper presents a mathematical model of chromium and lead leaching from internal pipe cement coatings to drinking water soon after renovation. Among other things, the model takes into account the adsorption of heavy metals inside porous cement coatings and diffusive transport, not only in coating capillaries but also in a water phase inside the pipe. Such phenomena have been ignored in other mathematical models simulating the leaching of heavy metals from pipe cement coatings. The proposed mathematical model was experimentally verified and high correlation between the results was observed. It has been proven that sorption inside the cement coating was responsible for the visible decrease of leaching in time, and that diffusive transport in the liquid phase inside the pipe influenced leaching speed. The experimental and numerical tests showed an initial sudden and significant increase to more than 10 μg/dm3, followed by a gradual decrease of chromium and lead leaching, reaching equilibrium concentrations close to 2.0 μg/dm3 after about 20 days. Enlargement of hydraulic pressure inside the pipe did not affect the initial increase of heavy metal concentrations in the water phase, but accelerated their decrease. The use of a twice larger pipe diameter led to an almost twofold reduction of initial peak chromium concentration inside the pipe.