The 27-year-old Harman appeared in some of the most notorious Abu Ghraib prison photographs, including one in which she posed with a pyramid of naked detainees.
She could face a maximum of five-and-a-half years in prison. Harman faced charges of conspiracy, dereliction of duty and maltreatment of subordinates.
Six other Americans have made plea bargains to Abu Ghraib charges, and ringleader Charles Graner was convicted at a January court martial and was sentenced to 10-years.
Lynndie England, the most recognizable Abu Ghraib defendant, also reached a plea deal, but the judge threw it out in early May after Graner's testimony contradicted England's assertion that she knew her actions were wrong.
Before a verdict was reached, a letter from Harman to her roommate in Virginia suggested that the army reservist took pictures of detainee abuse at Abu Ghraib prison, because she was opposed to the treatment and wanted to document the wrongdoing.
"At first I thought it was funny, but these people are going too far,'' Harman wrote to Kelly Bryant in the 20 October 2003, letter, which was introduced in court during defence arguments at her trial on Monday.
"Kelly, it's awful. I thought I could handle anything, but I was wrong.''
"At first I thought it was funny, but these people are going too far''
Sabrina Harman in a letter to a friend on 20 October 2003
But in closing arguments before the verdict was announced, prosecutors said Harman and other guards on the night shift at Abu Ghraib conspired to mistreat the prisoners.
"They were all acting together for their own amusement,'' said Capt. Chris Graveline. "There was no justification for what they did that night.''
Graveline said the group took pictures of what they were doing "so they could remember that night, so they could laugh again at these men. ... There's nothing funny about what happened at Abu Ghraib.''
However, defence lawyer Frank Spinner said Harman was a novice soldier, who had no prison guard experience and who received virtually no training before going to work at the chaotic and overcrowded prison as part of the Maryland-based 372nd Military Police Company.
Graner had earlier been handed
down 10 years for his role
"Shame on the army for putting an ill-equipped, ill-trained junior specialist in a position where she had to challenge her (enlisted) leadership to do the right thing,'' he said after putting on a case that lasted only a few hours
Harman, a former pizza shop manager from Virginia, is the second soldier to be tried for allegedly mistreating prisoners at Abu Ghraib.
She is depicted in several of the most notorious photos taken at Abu Ghraib in late October and early November 2003, and she is accused of taking other pictures.