Sgt Santos Cardona, 32, was also found guilty of dereliction of duty, through failing to use his military working dog for authorised purposes.
But he was acquitted in the court martial of seven other counts of abusing prisoners at the jail outside Baghdad between late 2003 and 2004, including the more serious charge of directing his dog to bite another prisoner.
He now faces up to three-and-a-half years in prison, and also possible discharge from the army or reduction in rank.
The panel of four officers and three enlisted soldiers deliberated on the verdict for almost 17 hours 16-and-a-half-hours, after a court martial which saw a week of testimony. A two-thirds majority was needed for a conviction on any charge.
Cardona was convicted of aggravated assault on prisoner Kamel Miza'l Nayil by "unlawfully threatening him with a means of force likely to produce death or grievous bodily harm, to wit: an unmuzzled military working dog."
He is now the 11th low-ranking US serviceman or woman to be convicted in the Abu Ghraib scandal. Another Abu Ghraib dog handler, Sgt Michael Smith, 24, was jailed for six months in March and demoted to the rank of private.
Some critics of the US administration and the Pentagon have complained that no senior officers have been prosecuted over the affair.
Cardona's defence lawyers had argued that use of dogs to interrogate prisoners was condoned by superior officers.
Cardona was also cleared of using his unmuzzled Belgian shepherd dog for what prosecutors said was personal "entertainment" derived from terrifying prisoners into urinating and defecating on themselves. Sentencing procedures started immediately after the verdict was announced.
US President George W Bush last week called the Abu Ghraib issue the biggest mistake of the US's war in Iraq.
The Abu Ghraib scandal erupted in early 2004 after photographs were leaked to the press showing US guards mistreating and sexually humiliating prisoners. Some pictures showed naked inmates cowering in front of unmuzzled dogs.