The Justice Department says it is evaluating the measures. When California was considering legalization in 2010, Attorney General Eric Holder said it would be a "significant impediment" to joint federal and local efforts to combat drug traffickers.
Federal agents have cracked down on medical pot dispensaries in states where it is legal, including California and Washington. Individual pot users may not be immediately impacted, as authorities have long focused on dismantling trafficking operations.
Peter Bensinger, administrator of the Drug Enforcement Administration from 1976 to 1981, and other former DEA heads urged Holder to make more noise this year about the pot votes. Colorado was a critical state for President Barack Obama's re-election.
Now, he said, "I can't see the Justice Department doing anything other than enforce the law. There's no other out."
Brian Smith of the Washington State Liquor Control Board, which will implement the new law, said officials are waiting anxiously to find out what federal law enforcement authorities plan to do. "They have been silent," Smith said.
Both states will have about a year to come up with rules for their legal pot systems.
In Mexico, which produces much of the pot that gets into the U.S. and where cartels and the government are embroiled in a yearslong deadly battle, the man in charge of Enrique Pena Nieto's presidential transition said the administration opposed legalization.
"These important modifications change somewhat the rules of the games in the relationship with the United States," Luis Videgaray told Radio Formula.
A former high-ranking official in the country's internal intelligence service who has studied the potential effects of legalization said he was optimistic that the measures would damage the cartels, possibly cutting profits from $6 billion to $4.6 billion.
Alejandro Hope, now an analyst at the think tank Mexican Competitiveness Institute, said among the complicating factors could be whether a strong U.S. crackdown on legal pot could negate all but the smallest effects on the cartels.
In Seattle, John Davis, a medical marijuana provider, called passage of the state's measure "a significant movement in the right direction." But he said he expected some confrontation with federal authorities.
"This law does not prevent conflicts," he said, adding that its passage "will highlight the necessity to find some kind of resolution between state and federal laws."
Associated Press writers Nicholas K. Geranios in Seattle, Pete Yost and Alicia Caldwell in Washington, and Michael Weissenstein in Mexico City contributed to this report.
Find Kristen Wyatt at http://www.twitter.com/APkristenwyatt .