During a swearing-in ceremony Monday, the new sheriff called for cooperation with the public, other leaders and his staff.
"I will earn your faith and earn your trust and we will get things accomplished," Trenary said.
Trenary pointed to the arrest of a suspect in the June 1 killing of 15-year-old Molly Conley in Lake Stevens as an example of the important work his office is called on to perform.
"Our people will not rest until justice is served," he said.
Snohomish County Superior Court Judge Michael Downes administered the oath of office to Trenary around 1 p.m. at a packed ceremony at the county campus.
Changes within the command staff are already in place.
They include naming Brent Speyer as the new undersheriff. Speyer served most recently as a lieutenant overseeing investigations, including the Conley shooting probe.
Former Undersheriff Tom Davis will serve as a captain overseeing special operations.
Trenary has worked in law enforcement for more than 25 years and for Snohomish County since 1991. Until becoming sheriff, he had been the captain of the sheriff's office north precinct. From 2008 to 2012, he was the police chief for Stanwood, which contracts with the county for police services.
Trenary's past positions have included working as a sheriff's office training manager and patrol supervisor. He's a former leader of the union that represents sheriff's deputies.
Several area police chiefs have long known Trenary and look forward to his tenure.
"His hallmark is that he's a people person," Edmonds Police Chief Al Compaan said.
"He's great with his troops, he's great with other law enforcement agencies in the county and he's great with the community."
Everett Police Chief Kathy Atwood, who has known Trenary for more than 20 years, expects to continue the close working relationship between Everett police and county deputies.
"Our guys do work really well together," Atwood said.
The sheriff's job became vacant June 3 when John Lovick resigned to accept the position of county executive.
The County Council on July 1 voted 5-0 to appoint Trenary as sheriff. The swearing-in ceremony was postponed a week because Trenary was out of town on a trip planned last year.
Council Chairwoman Stephanie Wright said she was impressed with Trenary's knowledge of the county budget and his experience with municipal issues in Stanwood.
"It really is a county team, especially when it comes to the budget," she said.
Trenary, 47, of Stanwood, is married to Vicki Trenary, an elementary school teacher in the Marysville School District. They have lived in Snohomish County for more than 20 years and have two daughters.
The sheriff is responsible for law enforcement in unincorporated Snohomish County. The office provides that service under contract in several communities, including Snohomish, Stanwood and Sultan. The sheriff's office also runs the county jail.
Combined, those operations have a staff of about 700 and an annual budget upwards of $100 million.
A citizens commission has set the sheriff's annual pay at about $123,000, which is less than what many of the department's top brass earn.
Voters will cast ballots in a 2014 special election to determine who serves out the remaining year left on the sheriff's term. A regular election for the four-year term is scheduled in 2015. The post is nonpartisan.
Trenary plans to run in those elections. So does sheriff's detective Sgt. Jim Upton, the other sheriff candidate interviewed by the County Council.
Noah Haglund: 425-339-3465; email@example.com.
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