Donald Clark was hired by the Department of Energy as the site manager to Emmett Mountain to replace the previous site manager Wesley Wiskowski who mysteriously disappeared after traveling to Washington, D.C. Donald didn't question the disappearance of the previous site manager because the promotion was a step up for him.
As his first order of business as site manager, Clark surveyed the entire interior structure of Emmett Mountain to find that the support structures were made of wood and would easily collapse with dry rot (which happened with a previous cell).
He ordered reconstruction using metal supports and also traveled to Washington, D.C. in hopes of talking to his superiors. Donald Clark also mysteriously disappeared and was replaced by Amelia Reynolds.
Donald Clark is mentioned only in Fallout 76.