Use the noun mandarin when you're talking about a powerful member of a government, company, or cultural group. In other words, don't mess with the mandarin.
You can use mandarin to mean "bureaucrat," or an official who tends to make things complicated and who wields a lot of power. Another kind of mandarin is a respected cultural or academic figure: "My favorite philosophy professor is considered a mandarin on campus." Mandarin with a capital "M" refers to the language spoken in China. Originally, this word meant simply "Chinese official," from the Hindi root word mantrī, "counselor."