Colorado and Washington have became the first US states to legalise the possession and sale of marijuana for recreational use in defiance of federal law.
The ballots set the stage for a possible showdown with the Obama administration.
But another ballot measure to remove criminal penalties for personal possession and cultivation of recreational cannabis was defeated in Oregon, where significantly less money and campaign organisation was devoted to the cause.
Supporters of a Colorado constitutional amendment legalising marijuana were the first to declare victory, and opponents conceded defeat, after returns showed the measure garnering nearly 53% of the vote versus 47% against.
"Colorado will no longer have laws that steer people toward using alcohol, and adults will be free to use marijuana instead if that is what they prefer.
"And we will be better off as a society because of it," said Mason Tvert, co-director of the Colorado pro-legalisation campaign.
The Drug Policy Alliance, a national advocacy group that backed the initiatives, said the outcome in Washington and Colorado reflected growing national support for liberalised cannabis laws, citing a Gallup poll last year that found 50% of Americans favoured making it legal, versus 46%.
Supporters of Washington State's pot legalisation initiative declared victory after the Seattle Times and other media projected a win for marijuana proponents.
Early returns showed pro-legalisation votes led with 55% versus to 44% opposed with about 60% of ballots tallied in the state's all-mail-in election system.
The outcomes in Colorado and Washington, which already have laws on the books legalising marijuana for medical purposes, put both states in further conflict with the federal government, which classifies cannabis as an illegal narcotic.
The US Department of Justice reacted to the measure's passage in Colorado by saying its enforcement policies remain unchanged, adding: "We are reviewing the ballot initiative and have no additional comment at this time."
Separately, medical marijuana measures were on the ballot in three other states, including Massachusetts, where CNN reported that voters approved an initiative to allow cannabis for medicinal reasons.
Supporters there issued a statement declaring victory for what they described as "the safest medical marijuana law in the country." Seventeen other states, plus the District of Columbia, already have medical marijuana laws on their books.
A measure that would have made Arkansas the first state in the south to legalise marijuana for medical purposes appeared headed for defeat by 51% to 49% with about 80% of the vote tallied.