For reporter Carol Leonnig, patience, timing and some extraordinary effort paid handsome returns.

The Washington Post journalist spent months gaining the trust of normally reticent Secret Service agents to produce a series of bombshell stories last year about lapses in presidential security. In the wake of her reporting, which shook the faith in what was commonly viewed as an elite and selfless agency, President Obama replaced more than half of the service’s senior leadership, including its director.

On Monday, Leonnig, 49, was awarded journalism’s highest honor, the Pulitzer Prize, for her revelations about the Secret Service. She was recognized in the national reporting category in an announcement by Columbia University in New York, which administers the awards.

The New York Times won three Pulitzers, two of them for reporting on the Ebola crisis in Africa, and the Los Angeles Times was a double winner, collecting Pulitzers in criticism and feature writing. The New York Times staff was recognized in the international category for its Ebola stories and for the feature photography of a free­lancer, Daniel Berehulak, who documented the epidemic in West Africa. Times reporter Eric Lipton won in the investigative reporting category, which had a second winner, the Wall Street Journal staff. Lipton won for stories about lobbyists’ influence over state attorneys general. The Journal’s Pulitzer — its first since Rupert Murdoch’s News Corp. took over the paper in 2007 — was about flaws in the Medicare system.

The Washington Post's Carol D. Leonnig was awarded the Pulitzer Prize for National Reporting on Monday for uncovering security lapses within the Secret Service. Executive Editor Martin Baron, Leonnig and others celebrated with speeches in the newsroom. (The Washington Post)

The Post and Courier of Charleston, S.C., won the gold medal for public service, arguably the most prestigious of the 14 journalism categories, for a five-part series titled “Till Death Do Us Part.” The stories, which the Pulitzer judges said were “riveting,” explored the state’s lax enforcement and weak penalties for domestic abuse. It spurred state legislators to fast-track ­legislation to overhaul domestic-violence laws.

Leonnig’s path to the Pulitzer began in early 2012 when she began reporting on the aftermath of a scandal involving 12 Secret Service agents. The agents brought prostitutes to their hotel rooms in Cartagena, Colombia, while making security arrangements for Obama’s visit to the city.

Leonnig turned up more than just embarrassment among the agents she contacted about the episode; several expressed deeper concerns about the Secret Service’s ability to guard the president, his family and senior members of the government.

“They saw serious security risks,” said Leonnig, a 15-year veteran of the newspaper. “There was a lot of distrust among the rank-and-file agents about the people who were running the agency.”

So Leonnig began collecting names, developing contacts and gaining the trust of agents, who rarely interact with reporters and risk being fired for doing so. “It was like the [shampoo] commercial,” Leonnig said. “One person told two friends [about her] who told two friends and so on.” Eventually, she built “a small town” of inside sources.

Wary that her phone and ­e-mail contacts with agents could be unveiled, Leonnig and her sources began using code names to refer to officers and officials. Leonnig’s concern grew out of her reporting in 2013 on the National Security Agency’s extensive surveillance programs; she was part of a team of Post reporters who won a Pulitzer for those revelations last year.

Her patience eventually produced a stack of full notebooks and a long list of story ideas. Leonnig and her editors were most intrigued by a tip about the Secret Service’s fumbling response to an attack on the White House in November 2011.

Leonnig spent much of last summer piecing together details of the incident, in which a gunman, Oscar R. Ortega-Hernandez, fired seven shots at the mansion, damaging parts of it but causing no injuries. Her investigation — in which she unearthed “a treasure trove” of official documents that prompted a late rewrite — revealed that the Secret Service had been slow to respond to the gunfire; one agent assumed it was merely the backfire from a construction vehicle and ordered others to stand down.

The story, published Sept. 27, also reported that it took the Secret Service four days to realize that shots had hit the White House. The discovery was prompted by a housekeeper, who noticed broken glass and a chunk of concrete on the floor.

It was just the beginning of a series of rapid-fire scoops and important developments.

Two days later, Leonnig revealed that an armed man, Omar Gonzalez, had overpowered a Secret Service agent and made it deep inside the White House after jumping the perimeter fence on Sept. 19. The Secret Service had initially said that Gonzalez had been arrested just inside the front door.

The next day, Leonnig and the Washington Examiner newspaper revealed another major security breach: An armed security guard with an arrest record was permitted to share an elevator with Obama during a presidential visit to Atlanta in mid-September.

Secret Service Director Joseph Clancy subsequently testified that “a lack of due diligence” led to the incident and that the agency that day had “failed our procedures.” Obama said later that he learned of the episode only from the newspaper accounts.

(An initial Post report that relied on sources in describing the security guard as a felon was later corrected. The correction was disclosed to the Pulitzer jurors.)

The stories raised serious questions about the agency’s leadership and led to bipartisan criticism of it during a House hearing on Sept. 30. A day later, Obama reversed his support for Secret Service Director Julia Pierson, prompting her resignation.

“It takes a gutsy journalist to dig beneath the surface of an organization like the Secret Service,” said The Post’s executive editor, Martin Baron. “It’s anything but transparent. It deflects and stonewalls inquiries from the press. It closes ranks when challenged, determined to maintain its Hollywood image. But Carol documented startling weaknesses and failures at the agency that had gone unaddressed and undisclosed. Her work led to sweeping leadership changes at the agency and will translate into greater safety for the president and others protected by the Secret Service.”

The Pulitzer capped a string of journalism honors for Leonnig, a native of Upper Marlboro in Prince George’s County. She won the prestigious George Polk Award for her Secret Service reporting in February and was a finalist for the Selden Ring Award for investigative journalism the same month.

“I feel a little strange being the centerpiece of [the Pulitzer] because it involved so many other journalists and involved so many risks by public servants who entrusted me with their confidence and their information,” she said. “Because of them, we were able to peel back the layers of this secretive agency and get the administration to focus on the very strong likelihood that the agency was not all it pretended to be and that posed serious risks for the president.”

Leonnig hasn’t heard from Obama, but that may change later this week. On Saturday, she’ll accept an award from the White House Correspondents’ Association at its annual dinner. The honor includes a private reception with the president beforehand.

Among the other journalism winners announced Monday:

● Los Angeles Times television critic Mary McNamara was honored in the criticism category and Diana Marcum won in feature writing for stories about California’s drought.

● The breaking news award went to the Seattle Times’ staff for its digital account of a landslide that killed 43 people and for its follow-up reporting.

● Bloomberg News reporter Zachary R. Mider won the explanatory reporting prize for “a painstaking, clear and entertaining explanation” of how so many U.S. corporations dodge taxes.

● Three reporters from the Daily Breeze of Torrance, Calif., (Rob Kuznia, Rebecca Kimitch and Frank Suraci) shared the prize for local reporting for articles about corruption in a local school district.

● The St. Louis Post-Dispatch’s photography staff won the breaking news photography award for its images of the events surrounding the unrest in Ferguson, Mo., following the police shooting of 18-year-old Michael Brown.

● The commentary prize was awarded to Lisa Falkenberg of the Houston Chronicle for columns about grand-jury abuses.

● The editorial cartooning winner was Adam Zyglis of the Buffalo News.

● Kathleen Kingsbury of the Boston Globe won the editorial writing prize for her work about fast-food workers and income inequality.