Systemic Risk Mitigation Act of 2020 This bill automatically designates certain nonbank financial companies as systemically important, which subjects these companies to certain prudential standards, including capital and liquidity rules, annual stress tests, and living wills requirements. This designation may be rescinded by the Financial Stability Oversight Council upon further review. The bill provides the council with rulemaking authority regarding an activity or practice that increases risk to the financial system. The bill creates the Climate Change Subcommittee within the council to address the risk climate change poses to the financial system. The bill also revises administrative provisions of the council, including those regarding staffing, funding, and meetings.