California, 1913

The first state marijuana prohibition law was passed in California in 1913. The law received no public notice in the press. It was passed as an obscure technical amendment by the State Board of Pharmacy, which was then leading one of the nation's earliest and most aggressive anti-narcotics campaigns. Prior to the passage of the law, there was no indication that cannabis was a problem in California. The Origins of Cannabis Prohibition in California from California NORML

Utah, 1914

Utah outlawed marijuana in 1914. A number of Mormons moved to Mexico when polygamy was outlawed in Utah in 1910. When they returned to Utah, marijuana was one of the things they brought back with them. Marijuana was outlawed with a number of other common vices as part of Mormon religious prohibitions enacted into law.

Other state laws 1915-37

Marijuana was outlawed in 30 states by 1930. There were two primary reasons for the laws.

In the southwestern states, marijuana was outlawed because of racial prejudice against the Mexicans who used it. As one Texas legislator said, "All Mexicans are crazy and this stuff (marijuana) is what makes them crazy."

In the other states, it was outlawed because of fears that heroin addiction would lead to the use of marijuana - exactly the opposite of the modern "gateway" myth.

The Marihuana Tax Act of 1937

Marijuana was outlawed at the national level in the US by the Marihuana Tax Act of 1937. (Note that the US Government has traditionally used the spelling "marihuana".) There were a number of reasons given for its passage in the congressional testimony. The quotes included:

Two weeks ago a sex-mad degenerate, named Lee Fernandez, brutally attacked a young Alamosa girl. He was convicted of assault with intent to rape and sentenced to 10 to 14 years in the state penitentiary. Police officers here know definitely that Fernandez was under the influence of marihuana. But this case is one in hundreds of murders, rapes, petty crimes, insanity that has occurred in southern Colorado in recent years.
I wish I could show you what a small marihuana cigaret can do to one of our degenerate Spanish-speaking residents. That's why our problem is so great; the greatest percentage of our population is composed of Spanish-speaking persons, most of who are low mentally, because of social and racial conditions.
Did you read of the Drain murder case in Pueblo recently? Marihuana is believed to have been used by one of the bloody murderers.
. . .a boy and a girl . . . lost their senses so completely after smoking marihuana that they eloped and were married.
It is commonly used as an aphrodisiac, and its continued use leads to impotency.
Practically every article written on the effects of the marihuana weed will tell of deeds committed without the knowledge of the culprit, while he was under the influence of this drug. . . . "A man under the influence of marihuana actually decapitated his best friend; and then, coming out of the effects of the drug, was as horrified as anyone over what he had done" (9). Then we have the case of a young boy in Florida. The story runs as follows: "A young boy who had become addicted to smoking marihuana cigarettes, in a fit of frenzy because, as he stated while still under the marihuana influence, a number of people were trying to cut off his arms and legs, seized an axe and killed his father, mother, two brothers and a sister, wiping out the entire family except himself." (10)

From Additional Statement of Harry Anslinger from Transcripts of the Congressional Hearings for the Marihuana Tax Act

There was no credible evidence that any of these statements were actually true.

Source: http://druglibrary.org/schaffer/library/mj_outlawed.htm