The Obama administration under Napolitano's direction has greatly increased law enforcement presence along the southern border with Mexico -- and has ramped up deportation efforts -- while proposing a set a reforms being hotly debated in Congress.
"Over the past four and a half years, we have invested historic resources to prevent illegal cross-border activity," she said. "Our borders are now better staffed and better protected than at any time in our nation's history, and illegal crossings have dropped to near-40-year-lows."
Napolitano called on Congress to act on behalf of the so-called Dreamers, children who were brought to the United States at a young age and have grown up without proper documentation but have only known life in America.
She said Congress has had the opportunity to "set commonsense immigration enforcement priorities, with a focus on criminals, national security and public safety threats," but has failed to do so.
"Congress had a chance to give these so-called Dreamers a way to stay in our country through the Dream Act, but unfortunately, that legislation failed to garner the 60 votes needed for cloture, falling just five votes short, despite strong bipartisan support," Napolitano said.
Instead, she said she used her prosecutorial discretion first at the Obama administration's urging and later through an executive order, to give those who would be covered under the Dream Act more time to stay in the country.
In addition to immigration, Napolitano reflected on her agency's response to Hurricane Sandy and the Deepwater Horizon oil spill in the Gulf of Mexico -- one of the largest natural disasters and one of the largest ecological disasters in U.S. history.
"What you do here matters to the lives of people all across our great nation, and your decisions affect them in direct, tangible ways," she said to DHS workers. "You make sure their families are safe from terrorist threats, that their local first responders have equipment and training and funding, and that when disaster strikes, people who have lost everything are given food, shelter and hope."
Napolitano, who has run the Department of Homeland Security for more than four years, is stepping down next week to lead California's university system. Her replacement has not yet been named.
Notable deaths of 2014 [PHOTOS]