Lanthanum hexaboride (LaB6) is well-known for its established physical properties, such as its incredible hardness and thermionic emission. On the nanoscale, LaB6 is gaining in popularity as an IR-active plasmonic material. However, while researchers are learning how to control the synthesis and tune the plasmon of these particles to meet desired application needs, very little is known about the basic chemical mechanism behind its formation. The assumptions that high temperatures and pressures were required to drive the reaction of LaB6 have been proven false in recent years, raising even more questions with regard to what conditions are really mandatory when designing a synthetic protocol. This work reports the use of in-situ diffraction experiments to help determine how lanthanum chloride and sodium borohydride precursors interact with and change one another in the LaB6 synthesis, and highlights the importance of the halogen in the reaction scheme. In understanding the driving forces and mechanism behind LaB6 formation, it may be easier for researchers to predict and control the particle shape, size, and even electronic properties of their hexaboride materials.