(CNN) -- Environmental Protection Agency Administrator Lisa Jackson will step down after the president's January State of the Union speech, the agency said Thursday.
Jackson thanked President Barack Obama for having placed her in the post four years ago and said she leaves the EPA "confident the ship is sailing in the right direction."
It is not uncommon for agencies to change leadership when new presidential terms begin. Jackson said she is "ready in my own life for new challenges, time with my family and new opportunities to make a difference."
In a statement, she recalled the goal she set at the beginning of her tenure to focus on climate change, but not to ignore other issues such as air pollution, toxic chemicals and waste-site cleanup issues.
"The way to transform this country and make it ready for the next 100 years is through new legislation that addresses energy, that deals with climate change, that deals with the issues that people are so scared of," Jackson told CNN at the beginning of her tenure as administrator.
The Obama administration's record on the passing tough environmental laws has been mixed.
Under Jackson's leadership, the EPA created new federal standards on toxic pollutants and mercury emissions from coal power plants. It was the first time that U.S. coal- and oil-fired power plant operators were required to limit their emissions of mercury and other hazardous pollutants.
Jackson also vigorously defended her agency against a 2010 bill that would have stopped the EPA from regulating carbon emissions. She wrote a column where she accused the bill's backers of siding with "big oil companies and their lobbyists" in an effort to "take away EPA's ability to protect the health and welfare of Americans from greenhouse gas pollution."
The bill was defeated in the Senate.
"Over the last four years, Lisa Jackson has shown an unwavering commitment to the health of our families and our children," Obama said in a statement. "Under her leadership, the EPA has taken sensible and important steps to protect the air we breathe and the water we drink, including implementing the first national standard for harmful mercury pollution, taking important action to combat climate change under the Clean Air Act, and playing a key role in establishing historic fuel economy standards that will save the average American family thousands of dollars at the pump, while also slashing carbon pollution."
During this year's presidential campaign, GOP candidate Mitt Romney accused Jackson and two other Obama appointees of pursuing policies that drive up gasoline prices. He called for her firing.
There were times when Obama himself became an obstacle to Jackson.
In September 2011, Obama ordered Jackson to scrap a plan for reducing ozone pollution. At the time, the president stressed his environmental record, but said the additional regulations would create uncertainty that could slow down the economic recovery.
The move was criticized by environmentalists who accused Obama of siding with corporate polluters over the public's health.
The EPA had argued that increasing the standard would save 4,300 lives per year, prevent 7,000 hospital visits and avoid 2.6 million missed days of work or school.
Amid rumors that Jackson would leave the agency, a grassroots movement emerged online, asking her to remain on the job, which she did.
Jackson said that her background as a mother, a graduate from Princeton and Tulane universities and resident of New Orlean's 9th Ward allows her to tackle the issues facing the environment.
She was the first African-American to head up the EPA, a distinction she gave weight to as she undertook her work.
"I don't think of it every moment," she told CNN as she entered the job. "(But) what I hope that we see at the end of this are activists who look like me -- activists who represent the future demographic of our country, because that's who's going to be the EPA in the future."