Commander Rivera began his career with MPD in 1992. After graduating from the Academy, he was assigned as an officer in the Fourth District until 1997 and then the Police Recruiting Unit. In 1999, he was promoted to sergeant and returned to the Fourth District. He was later assigned to the Internal Affairs Division where he investigated allegations of misconduct and corruption practices by members of the department. Promoted to lieutenant in February 2007, he served as a Detective-Lieutenant on the Force Investigations Team, leading investigations into Serious Uses of Force and In-Custody Deaths by members of the department. After that, he served as a lieutenant in the Narcotics and Special Investigations Division; detective lieutenant in the Criminal Investigations Division; PSA lieutenant in Seventh District and administrative lieutenant in the Special Operations Division. In April of 2016, he was promoted to captain and managed the Marshall Heights Area during the 2016 Summer Crime Initiative and also worked in the Professional Development Bureau. He was named Captain of the Year in 2016. Guillermo Rivera was promoted to the rank of commander in April 2017 and assumed command of the Special Operations Division.
In January of 2021 Commander Rivera was reassigned to the newly created Intelligence Division which also oversees MPD members of the FBI’s Joint Terrorism Task Force, Washington, DC, Field Office.
Rivera retired from the Metropolitan Police Department in December 2023.