"mandatory" from Latin mandātor (“someone who orders”), compare: "commandant" "required" from Latin requīrō (“i seek, ask for”), compare: "request" and "aquire

"mandatory" from Latin mandātor (“someone who orders”), compare: "commandant" "required" from Latin requīrō (“i seek, ask for”), compare: "request" and "aquire" Today the words are mostly interchangeable, but mandatory is more often used in context of laws and government while "required" is seen mostly in tech and engineering.