Jackson, 50, a chemical engineer by training, did not point to any particular reason for her departure. Historically, Cabinet members looking to move on will leave at the beginning of a president's second term.
"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."
Jackson experienced another big setback last year when the administration scrubbed a clean-air regulation aimed at reducing health-threatening smog. Republican lawmakers had been hammering the president over the proposed rule, accusing his administration of making it harder for companies to create jobs.
She also vowed to better control toxic coal ash after a massive spill in Tennessee, but that regulation has yet to be finalized more than four years after the spill.
Jackson had some victories, too. During her tenure, the administration finalized a new rule doubling fuel efficiency standards for cars and light trucks. The requirements will be phased in over 13 years and eventually require all new vehicles to average 54.5 mpg, up from 28.6 mpg at the end of last year.
She shepherded another rule that forces power plants to control mercury and other toxic pollutants for the first time. Previously, the nation's coal- and oil-fired power plants had been allowed to run without addressing their full environmental and public health costs.
Jackson also helped persuade the administration to table the controversial Keystone XL pipeline, which would have brought carbon-heavy tar sands oil from Canada to refineries in Texas.
House Republicans dedicated much of their time this past election year trying to rein in the EPA. They passed a bill seeking to thwart regulation of the coal industry and quash the stricter fuel efficiency standards. In the end, though, the bill made no headway in the Senate. It served mostly as election-year fodder that appeared to have little impact on the presidential election.