When was the last time in the U.S. that someone was executed under a government-imposed death penalty but later exonerated?
As of April 2020, the last person who was executed under the death penalty and later exonerated was put to death in 1939, or 81 years ago. An exoneration is an official act by the government declaring that a previously convicted person is innocent. Some death penalty opponents assert that innocent people have been executed in recent times but that "there is no way to tell how many" because courts and defense attorneys do not pursue exonerations after the defendant dies. In reality, advocacy groups and media outlets investigate post-execution innocence cases, and in some cases, governments participate. Thus, the person who was put to death in 1939 was exonerated in 2011. Other examples of such cases are detailed at the link below.