Mandatory Minimum Reform Act of 2017 This bill amends the Controlled Substances Act and the Controlled Substances Import and Export Act to eliminate most drug offense mandatory minimum prison terms imposed on a defendant who imports, exports, manufactures, distributes, or possesses with intent to distribute a controlled substance. Additionally, it eliminates drug offense mandatory minimum prison terms for simple possession, distribution to an individual under age 21 by an individual at least age 18, distribution or manufacture in a school zone, and use of an individual under age 18 to distribute. Finally, the bill requires prior written approval of the Attorney General to prosecute a defendant for certain drug offenses that involve low-level drug quantities (e.g., less than one kilogram of heroin or less than 500 grams of cocaine-based substances).