MILL CREEK, Wash.—A drug task force in Washington's Snohomish County has historically been funded in part by cash, cars, houses and other assets seized from marijuana purveyors. But with recreational pot becoming legal in the state, this funding is going up in smoke.

Snohomish's 22-officer drug-fighting operation, one of 19 such task forces in the state, brought in about $200,000 from forfeitures in marijuana cases in 2012—15% of its funding that year; the haul has exceeded $1 million in years past. The task force has a piece of land, seized from a pot grower, where it stores seized vehicles awaiting auction and trims with a riding mower confiscated in a drug bust.

The county's task force has already slashed its projected funding for this year by more than 15%, partially because of a decline in revenue from asset forfeitures in pot cases, said task force Commander Pat Slack. That will mean less money for overtime, training and new equipment, said Mr. Slack, a vocal opponent of legalization.

With marijuana legalized for those at least 21 years old in Washington later this year and in Colorado as of Jan. 1, law-enforcement agencies in those states expect to lose millions in revenue gained from assets seized from growers and dealers.