Thompson's family said he died at Memorial Sloan Kettering Cancer Center.
"Ken was a dedicated public servant who embodied the highest principles of the law, and his grand presence will be sorely missed," said Gov. Andrew Cuomo. He said flags would be flown at half-staff on Monday in tribute to Thompson.
Governor Andrew Cuomo:
I am profoundly saddened to learn of the sudden passing of Brooklyn DA Ken Thompson, a dedicated public servant. He will be sorely missed.— Andrew Cuomo (@NYGovCuomo) October 10, 2016
When he took office, Ken became the first African-American to serve as Brooklyn DA. My thoughts and prayers go out to the Thompson family.— Andrew Cuomo (@NYGovCuomo) October 10, 2016
Mayor Bill de Blasio:
Thompson had announced last Tuesday that he would be taking absences from his position to undergo treatment for cancer, saying, "As a man of intense faith, I intend to fight and win the battle against this disease." He didn't specify what kind of cancer he had.
Thompson defeated long-standing office holder Charles Hynes to take over the district attorney's office in January 2014. In his tenure, the office expanded an initiative that reviews cases for potential wrongful convictions. He also prosecuted Peter Liang, a New York Police Department officer who was tried in the death of an unarmed black man in a housing project stairwell. Liang was found guilty of manslaughter, but a judge downgraded it to criminally negligent homicide. Thompson decided against seeking prison time in sentencing.
"He was a giant among those seeking to reform the criminal justice system and we are all privileged to have worked under his transformative leadership these past three years," said Chief Assistant District Attorney Eric Gonzalez.
Thompson also worked in private practice. He represented Nafissatou Diallo, the hotel maid who accused former International Monetary Fund leader Dominique Strauss-Kahn of sexual assault in 2011.
During his time as a federal prosecutor, he prosecuted the case against a police officer accused in the 1997 assault of Abner Louima in a precinct bathroom.
Thompson is survived by his wife and two children, as well as his mother, father, brother and sister.