The legislation was passed by Congress last week and ratified by a majority of Mexican states.
Enrique Pena Nieto says the reform will help Mexico attract the investment needed to boost its falling oil output.
The opposition says it damages national interests.
The legislation changes three articles in the Mexican constitution and allows foreign investment in oil, gas and electricity.
Private companies will be allowed to sign contracts to drill for oil and gas with state-controlled firm Pemex, which will get a share of the profits.
“This is the beginning of a new history for our country. We have opened the doors for a better future for all,” said Enrique Pena Nieto.
Oil production in Mexico has dropped from 3.4 million barrels per day in 2004 to the current rate of 2.5 million barrels per day.
Opposition legislators attempted to stop last week’s vote by all means, including piling up chairs to block the entrance to the Congress building.
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