By 2000, the number had more than doubled to 31 percent and has been rising ever since.
In 1969, when Gallup first conducted a poll on the topic, merely 12 per cent respondents supported the drug. It increased slightly to 28 percent during the mid-'70s, but fell back down during the "Just Say No" anti-drug era of the '80s and wavered around 25 percent in the '90s.
As Gallup notes, support for legalization has soared in the past decade among almost every demographic group.
A national poll released last week by the Pew Research Center also found support is growing, with 57% of Americans saying they think the use of marijuana should be made legal, up from 53% previous year. Support was weakest among Republicans (42 percent) and those age 55 or older (45 percent).
In 2012, Colorado and Washington legalized recreation marijuana use-since then Alaska, Oregon, and the District of Columbia have all followed suit.
Tom Angell, founder of the pro-legalization group Marijuana Majority, said the Gallup poll was particularly encouraging for the future of legal weed because it showed that 77 percent of people under 34 are on board with changing the law. It wasn't until 2013 that a majority of Americans said they thought marijuana should be legalized.
Gallup finds a rise in support for marijuana legalization among every political affiliation.
The year of 2013 marked the first time when more than 50 per cent of people supported legalization of marijuana.
The news comes as voters in five states begin to vote on ballot initiatives to regulate and tax marijuana for adult use.
Perhaps realizing that the argument against legalization per se is already a lost one, opposition campaigns are focusing on specific provisions within the ballot proposals that voters may object to.
Angell said a sweep on Election Day could push Congress to change federal marijuana laws, while defeats in just a few key states, like California and MA, have the potential to keep lawmakers reluctant to embrace legalization.
The poll, from Gallup, found that 60 percent of Americans now say that using marijuana should be legal.
The latest polling out of California suggest that the legalization measure is leading by a two-to-one margin.