Obama's signing of the bill in the Oval Office was open to news photographers but not the rest of the media. He made no remarks.
But as a longtime thorn for the black community, the matter is important to a key Obama constituency.
The quarter-century-old law that Congress changed with the new bill has subjected tens of thousands of blacks to long prison terms for crack cocaine convictions while giving far more lenient sentences to those, mainly whites, caught with powder.
However, the new law is not retroactive.
And it applies only to federal defendants, with no impact on state mandatory sentencing laws. Most drug arrests occur at the state level.
But Monica Pratt, spokeswoman for Families Against Mandatory Minimums, said the states have generally been ahead of the federal government in moving away from mandatory sentences in general.
And Bill Piper, director of national affairs for the Drug Policy Alliance, said most states already treat crack and powder cocaine the same. The significance of the new law, he said, is that prosecutors sometimes kick cases up to the federal level to get harsher sentences and that now will be less likely to happen.
The measure Obama signed changes a 1986 law that was enacted at a time when crack cocaine use was rampant and considered a particularly violent drug. Under that law, a person convicted of possessing five grams of crack cocaine session got the same mandatory prison term as someone with 500 grams - 100 times - of powder cocaine. The legislation reduces that ratio to about 18-1.
The bill also eliminates the five-year mandatory minimum for first-time possession of crack, the first time since the Nixon administration that Congress has repealed a mandatory minimum sentence. It would not apply retroactively.
In the 2008 campaign, Obama cited figures that blacks serve almost as much time for drug offenses - 58.7 months - as whites do for violent offenses - 61.7 months. He said the wide gap in sentencing "cannot be justified and should be eliminated."