The mechanism of the reaction between Mn and Se is shown to depend on the composition and the thickness of Mn and Se layers. Samples were prepared by modulated elemental reactants (MER) method. The elements were evaporated using either electron beam guns or effusion cells and the total amount of each element was controlled using quartz crystal monitors. Different ratios of the elements and different total amounts of the elements per repeating unit were experimentally explored. Near a one to one ratio of Mn and Se, α‐MnSe crystallizes upon deposition. Compositions near a one to two ratio of Mn to Se proceed through an amorphous intermediate for the layer thicknesses investigated, with MnSe2 crystallizing at 150 °C. Between these compositions, the two bilayer thicknesses explored evolve differently, with films with bilayer thicknesses near 0.5 nm nucleating MnSe, whereas films with bilayer thicknesses around 1 nm nucleating a mixture of MnSe and MnSe2. Samples around 80 % Se formed a previously unreported compound with a small monoclinic unit cell with lattice parameters a = 4.942(2) Å, b = 4.32(3) Å, c = 3.779(1) Å, and β = 90.13(3)°.