A modular exterior threshold and door sill assembly is provided for an entryway. The threshold and sill assembly includes an elongated sill assembly having at least one elongated channel. An elongated rail assembly is provided that extends at least a portion of the length of the elongated sill assembly. A portion of the elongated rail assembly is received within the elongated channel. The threshold and sill assembly includes at least one sill component connected to one of the elongated rail assembly and the elongated channel, wherein the at least one sill component includes at least one of a cover assembly, an astragal bolt receiver assembly, a mullion boot assembly, a side panel spacer assembly and an outswing rail assembly. The elongated rail assembly may be fixed or adjustable. The adjustable rail assembly may be adjusted using adjustors that are fixedly secured to the sill assembly.