The 42-year-old, who testified before the Senate Commerce Committee on Capitol Hill in Washington on Wednesday, said the thefts occurred during her almost five-year tenure and she wants to 'sincerely apologize to each and every one of our users'.
In an opening statement to the Senate Commerce Committee, Mayer apologized to Yahoo's users, blamed "Russian agents" for the breach and said that Yahoo quickly worked to protect user accounts and contact law enforcement.
One of those suspected hackers, Karim Baratov, has even been arrested and extradited to the US.
Mayer and Smith were flanked by officials from their former companies, as Congress sought to get to the bottom of how three billion global user accounts at Yahoo and 146 million American Social Security numbers and more at Equifax were stolen by hackers.
"I believe all companies, even the most well defended ones, could fall victim to these crimes", she said. Because of the information contained in the field, Mayer said the company was confident that the breach occurred in 2013.
"Unfortunately, while all our measures helped Yahoo successfully defend against the barrage of attacks by both private and state-sponsored hackers, Russian agents intruded on our systems and stole our users' data".
Executives from Yahoo and Equifax testified to Congress on Wednesday, apologizing for massive breaches affecting billions of people around the world. Wednesday's session marks a Washington debut for Paulino do Rego Barros, who took Equifax's helm in September.
The hearing comes after Equifax said failure to install a security update may have led to exposure of information of more than 145 million people in the U.S. and nearly 700,000 people in the UK.
Barros told the committee he has focused on improving customer service and revising the company's structure so that the company's chief security officer reports directly to him.
He said a federal law should replace that patchwork of laws.
Last month, Yahoo said the breach apparently exposed all its users at the time it occurred. Florida Sen. Bill Nelson asked Mayer.
"To this day, we have still not been able to identify the intrusion that led to the attack".
'We don't exactly understand how the act was perpetrated, ' which explains why there are some gaps in information, she said.