Uniform material distribution by atomic layer deposition (ALD) inside porous materials is needed in multiple applications, including batteries and catalysis. Attaining this uniformity is not trivial, diffusion within the porous network being one of the main limiting factors. This work used a fluidized bed atmospheric ALD reactor to coat millimeter-size mesoporous alumina spheres with platinum, using the process based on (methylcyclopentadienyl)trimethylplatinum [MeCpPtMe3] and oxygen. Using different exposure times and five reaction cycles, materials with platinum loading up to ∼4 wt% were prepared. The growth per cycle, expressed as average areal number density, was approximately 0.1 Pt atoms per nm2. Cross-sectional analysis done using low-energy ion scattering indicated that with increasing exposure time, platinum distribution evolved from egg-shell to macroscopic uniform distribution through the particles. Diffusion–reaction modeling was done to support the experiments and showed a saturation of the Pt weight loading after uniform distribution. This work shows that it is possible to get a uniform distribution of platinum through mesoporous particles with an aspect ratio on the order of 100 000 : 1, when the ALD process is properly optimized.