Elections have also occurred at federal, regional and local levels.
Lopez Obrador's victory was expected by many to further worsen Mexico's already frosty relations with the US, whose president, Donald Trump, has slammed the country over illegal migration and pressured it to pay for a border wall. Peña Nieto has been criticized by many who say he failed to adequately deal with crime, corruption and economic inequality.
Voter turnout was estimated at 63 per cent of the 89 million eligible voters in the country's biggest ever elections.
Younger voters have grown up surrounded by rampant corruption and drug violence. We report from Mexico City, where, despite the country losing their World Cup game to Brazil today, many have been celebrating the election result. Some people told CNN they were waiting for the match to finish before making their way to the polls.
The peso declined as much as 1.3 percent after results of the Sunday elections signaled his Morena party was set to enjoy broad support in Congress. In both of Lopez Obrador's previous two presidential losses he alleged fraud.
Lopez Obrador, a former Mexico City mayor, was greeted with rapturous cheers by supporters in the capital's Zocalo central square around midnight, while friends celebrated in his tiny hometown of Tepetitan, in the poor southern state of Tabasco.
Lopez Obrador's coalition, which is led by his Morena party, looked like it was also on track to snag several other key posts, including at least five of the nine governor's races that were on the ballot.
Carlos Urzua, Lopez Obrador's pick to be his finance minister when he takes office on December 1, told Reuters on Monday that the new government will aim to reform the pension system and to reduce costs at Pemex during its six-year term.
Finishing second again in 2012, he remained the most visible opposition leader and by this year had become the focal point of public frustration with the establishment's shortcomings.
Ricardo Anaya, National Action Party (PAN), 24 percent.
Governing party candidate Jose Antonio Meade earlier conceded victory to Lopez Obrador.
Voters decided in their millions to turn their backs on the two parties which have ruled Mexico for nearly 100 years, and finally give him a chance - sending his supporters into a frenzy.
"I don't know how you would apply it", said Juan Manuel Andazola, the editor of El Diario, one of Chihuahua's daily newspapers.
This campaign season in Mexico has been particularly violent.
The MXN= MXN=D2 strengthened over 2.5 percent to a more than one-month high in its biggest one-day percentage gain since early 2016, leading gains among major currencies as the USA dollar broadly weakened.
A political killing was reported even on election day.
The violence isn't limited to politicians, either. And in Michoacan state, polling places in a few villages were cancelled after some inhabitants said they didn't want elections involving political parties, which they mistrust.
In the following years, NAFTA not only transformed Mexico's economy but also affected its foreign policy towards the US.
- Lakers Dodged A Bullet Missing Out On Paul George
- DeMarcus Cousins joining Golden State Warriors
- Kane and England's new generation impress Granqvist
- Dwight Howard Wants To Come Back To The Lakers & Play Alongside LeBron
- Syrian army gains in southwest, rebels begin peace talks
- Colombia vs. England - Football Match Report
- Neymar leads Brazil into seventh straight quarterfinal
- Suspect in Maryland newspaper to appear in court on 5 murder charges
- The U.S. Chamber of Commerce Is Fighting Trump's Tariffs with Facts
- LeBron James signs for LA Lakers in $154MILLION DEAL