WASHINGTON — U.S. Secret Service Director Julia Pierson resigned under fire on Wednesday after a series of security lapses came to light that exposed gaping holes in the protective cocoon around President Obama.

Pierson, in her position for just 18 months, faced mounting calls from lawmakers to step down in the fallout from a September 19 incident in which an Iraq War veteran with a knife scaled the White House fence, sprinted across the lawn and got deep inside the mansion before an off-duty agent stopped him.

Pierson had told a congressional committee on Tuesday she took “full responsibility” for gaps in presidential security. On Wednesday, she offered her resignation in a meeting with Homeland Security Secretary Jeh Johnson, who accepted it.

White House spokesman Josh Earnest said Obama believed Pierson’s resignation was in the best interest of the agency, and that “recent and accumulating” reports of security lapses led the president to conclude new leadership was required at the Secret Service.

The development came a day after the disclosure that in a violation of protocol, an armed private security contractor with a criminal record rode on an elevator with Obama in Atlanta earlier this month and took pictures and video of the president on his phone.

The lapses, along with a 2011 incident in which seven gunshots were fired at the White House, had raised concerns across Washington that Obama was not as protected as he should be in an age of global tumult.

While all presidents have faced death threats, Obama, as the first African-American president, is believed to have received more than his predecessors.