Joe Arpaio, the controversial former sheriff of Maricopa County, Arizona, has been found guilty of criminal contempt – a federal offence.
He was found to have violated a judge’s 2001 order that he cease detaining migrants who are not suspected of having committed a state crime.
Judge Susan Bolton determined that by detaining those living in the US illegally, Mr Arpaio was acting as a de facto wing of the federal government.
He faces up to six months in prison.
However, lawyers say it is unlikely that he will ever serve time behind bars.
Mr Arpaio, 85, had boasted of being “America’s toughest sheriff” during his time as the elected lawman of Maricopa County, which includes the city of Phoenix.
He rose to national prominence due to his tough stance against illegal immigration. However, a judge reminded him during his trial that only federal officers have jurisdiction over immigration.
He had claimed that the judge’s injunction in 2011, which he was found to have violated, was vague and unclearly worded.
But a judge found on Monday that Mr Arpaio had understood the temporary injunction, which was later made permanent, and had deliberately violated it to score political points ahead of his re-election campaign in 2012.
He was known during his tenure as sheriff for sweeps of undocumented immigrants in Hispanic communities, and for detaining Spanish-speakers under suspicion of being undocumented migrants.
He also famously required his inmates to wear pink underpants and socks.
Mr Arpaio, in a statement, insisted that the judge who issued the ruling was biased, and said he would appeal to have a jury hear his case.
“Joe Arpaio is in this for the long haul, and he will continue his fight to vindicate himself, to prove his innocence, and to protect the public,” a statement issued by him reads.