This paper discusses the mechanical design and assembly of the Near Infrared Camera and Multi-Object Spectrometer (NICMOS) focal plane assembly (FPA). The FPA consists of a mercury-cadmium-telluride (MCT) detector array hybridized to a silicon multiplexer (MUX), a sapphire carrier, an alumina ceramic multi-layer board (CMLB) including electrical components, a base plate, and flex cables. The FPA is designed for the following conditions; (1) shock and vibration during launch, (2) Coefficient of Thermal Expansion (CTE) of dissimilar materials at cryogenic temperature, (3) outgassing limitations to meet NASA's specifications, and (4) optical assembly tolerances. Also, the FPA is designed to be easily integrated into its dewar with provisions for mechanical as well as optical alignment. The FPA is assembled by building up two subassemblies in a parallel path, and then integrating the two subassemblies with the flex cables for the final assembly. These procedures are described in this paper, including alignment tolerances required and measured.