Countering Weapons of Mass Destruction Act of 2018 (Sec. 2) This bill amends the Homeland Security Act of 2002 to establish in the Department of Homeland Security (DHS) a Countering Weapons of Mass Destruction Office, headed by an Assistant Secretary. The Assistant Secretary shall serve as the principal advisor to DHS on weapons of mass destruction matters and strategies, and on coordinating efforts to counter weapons of mass destruction. The office shall coordinate DHS strategy and policy to plan, detect, and protect against the importation, possession, storage, transportation, development, or use of unauthorized chemical, biological, radiological, or nuclear materials, devices, or agents. The Assistant Secretary shall establish the Securing the Cities program to detect and prevent terrorist attacks and other high consequence events utilizing nuclear or other radiological materials that pose a high risk to homeland security in high-risk urban areas. The bill establishes in the office a Chief Medical Officer who shall serve as the principal advisor to DHS on medical and public health issues. The DHS Under Secretary for Management shall be responsible for workforce-focused health and medical activities of DHS. DHS shall: (1) transfer to the office all personnel, budget authority, and assets of the Domestic Nuclear Detection Office and the Office of Health Affairs; and (2) provide a briefing and report to Congress on DHS chemical, biological, radiological, and nuclear activities.