Multnomah County Sheriff's Office (MCSO) serves the close to 700,000 residents of Multnomah County, Oregon, United States. Multnomah County Sheriff's Office is a County Law Enforcement agency that handles 9-1-1 calls and assists other city agencies such as Portland Police Bureau. The current Sheriff is Nicole Morrisey O'Donnell. The cities of Maywood Park, Wood Village, Fairview, and Troutdale contract out the law enforcement services of Multnomah County Sheriff's Office.

The sheriff is elected by popular vote, and oversees a budget of $118 million.

Multnomah County Sheriff's Office was established in February 1854, with William L. McMillen as the first sheriff; he served until 1862. In 1960, the Sheriff's Office created the River Patrol Unit, which has grown to be the largest river patrol division in the state.

In the past, as recently as the 1980s, the position was essentially the "top cop" in the county, performing law enforcement for the bulk of the county's population. As of 2009, however, as the unincorporated areas of the county have diminished, 85% of sheriff's office employees work in corrections rather than law enforcement.

On July 1, 2015, the Sheriff's Office began policing services for the city of Troutdale. In a 10-year agreement, the officers and civilian personnel of the Troutdale Police department were brought in as sworn deputies and employees of the Sheriff's Office. The agreement was reported to save the City of Troutdale over $900,000 per year over a ten-year period. The deal also included leasing of the Troutdale Police Community Center. The Law Enforcement patrol, property, and records divisions were also moved from the Hansen Building to this location in July, 2015.

The sheriff's office oversees a budget of $118 million.

In early 2020, the Sheriff's Office switched their emerald uniforms to black ones. Part of this change also included updating the vehicle livery and retiring their green vehicles.

In 2023, Nicole Morrisey O'Donnell became the first female Sheriff in the agency's history.

Overdoses among inmate escalated due to illegally smuggled drugs by inmates. In August 2023, a strip search was put back into place in response.

Former Sheriff Bernie Giusto resigned on July 1, 2008 after a state police standards board recommended that he lose his badge because of multiple issues including using an official vehicle for personal use, lying about a relationship he had with former Governor Neil Goldschmidt's wife while he was assigned to the governor's protective service branch in the mid-1980s, and allegations that he lied about his knowledge of Goldschmidt's illegal sexual relationship with a minor in the 1970s. In addition to ethics violations, Giusto was thought to have managed the county budget poorly, and when confronted about such issues once replied with "They're not my bosses; they're my bankers, I'm not gonna debate how I do spend my budget. That's why I'm independently elected." Days after making that comment, he closed the county's restitution center, which was a program that was just created that year.

In 2015, Sergeant Brent Ritchie worked with three other deputies to investigate use of force in the Sheriff's Office's jails. Their conclusion was that use of force was disproportionately used against Black inmates. Sheriff Dan Staton demoted Ritchie upon being shown the final report.

In 2019, a court rules that Staton retaliated against Ritchie and Ritchie was awarded $250,000 plus attorney's fees.

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