“There are no guilty men in prison” is an old prison adage; seemingly every prisoner claims that he or she is innocent and was wrongly charged and convicted. The saying has no particular author, but is commonly known by prison staffs, prosecutors and defense attorneys.
 
“Penitentiaries throughout the world are filled with men who protest their innocence. Weary and cynical wardens will tell that there are apparently no guilty men in prison” was cited in print in 1948.
 
 
26 February 1948, Hartford (CT) Courant, “New Films: ‘CALL NORTHSIDE 777’ At Loew’s Poli” by H.V.A., pg. 6:
Penitentiaries throughout the world are filled with men who protest their innocence. Weary and cynical wardens will tell that there are apparently no guilty men in prison. But in this country most of the men spending their time behind bars are there because there is where they belong.
 
15 August 1966, Boston (MA) Herald, Jimmy Breslin column, col. 4, col. 3
Then the daily, and virtually only, topic of prison conversation started. There are no guilty men in prison, Only men of towering innocence who have been wronged by justice.
 
Google News Archive
7 July 1974, Palm Beach (FL) Post-Times, “Letters to the Editor: Hearts Ought To Bleed For Victims,” pg. D2, col. 4:
The article merely points out the fact that there are no “guilty” men in prison. They are all “innocent victims of circumstance.”
(...)
Alan H. Wieler
West Palm Beach
 
Google Books
Understanding and Changing Criminal Behavior
By Michael J. Lillyquist
Englewood Cliffs, NJ: Prentice-Hall
1980
Pg. 55:
One occasionally hears a prison official remark that “there are no guilty men in prison,” at least by the prisoners’ own reckoning.
 
11 December 1988, Miami (FL) Herald, “The Contessa,” Tropic, pg. 4:
There is an old saying that there are no guilty men in prison.
 
Google News Archive
21 October 1991, Milwaukee (WI) Sentinel, “Many believe xx-shipbuilder Is Victim In Kickback case” by Eldon Knoche, pg. 1, col. 4:
Although it’s a common joke among jailers that there are no guilty men in prison because they all claim innocence, many who know Kerkman believe he may be that rare exception.
     
Google Books
Guilty Until Proven Innocent
By D. Corbett Everidge
Lincoln, NE: iUniverse
2007
Pg. 149:
It has often been said that “there are no guilty men in prison.” As a former probation officer, I have to say this is nothing more than a myth.