Most of us can't claim notorious criminals such as John Dillinger, Al Capone or Bonnie & Clyde in our family tree, but our ancestors may have been convicted and imprisoned for hundreds of lesser reasons just the same. State and federal penitentiaries and prisons, state archives and other repositories have put a wealth of records and databases online that may put you hot on your ancestor's trail. These online indices often include extra details from descriptions of the offense, to the inmate's place and year of birth. Mug shots, interviews and other interesting records may also be found in these databases of Historic U.S. Prison Records Online or Researching Criminal Ancestors in Britain.
While having these prison and inmate databases available online is a great starting point, most of the records beg that you dig further -- into correctional records, court records, jail logs, Governor's papers, records of the Secretary of State and/or Attorney General, etc. Historical newspaper accounts of the crime and conviction can also add substance to your family history.
Hundreds of thousands of other criminal records are also waiting to be discovered in state and university archives, county courts and other repositories. Your ancestor may not have been sent to San Quentin for murder, but you may be surprised to find a newspaper account of his being investigated for arson, or being arrested for a minor misdemeanor such as vagrancy, petty larceny, gambling or even making moonshine. Turn to finding aids for repositories such as the State Archives, the Family History Library Catalog or the local county historical society to learn what might be available offline for researching your own criminal ancestors.