We demonstrate the ability to control the placement of molecules within self-assembled films. We show quasi-two-dimensional mosaic structures within two-component self-assembled monolayers. These demonstrate new types of monolayer features: the domains of each component form molecular terraces, and the boundaries between these domains form molecular step edges. We find the molecular step edges are molecularly sharp and laterally epitaxial. The molecular step edges have the unique ability to expose a typically buried part of the larger molecule at the boundary. For the identical components in the crystalline monolayer, we can also distribute the molecules randomly. These structures were selected and created using simple controlled sequences of self-assembly and processing with different length alkanethiols on Au{111}. We have imaged these films using molecular resolution scanning tunneling microscopy.