Mandatory Minimum Reform Act of 2020 This bill eliminates most mandatory minimum prison terms for drug offenses involving the import, export, manufacture, distribution, or possession with intent to distribute a controlled substance. Additionally, it eliminates mandatory minimum prison terms for the following drug offenses: 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).