During her time at The Observer, she was a lead reporter on several investigative projects, including one involving Bank of America's use of federal funds to raze low-income housing near its corporate headquarters and another uncovering that Gov.

[9] In 2018, Leonnig was part of the team that won the Pulitzer Prize for national reporting as a contributor to 10 stories on the investigation of Russian interference in the 2016 election with The Washington Post.

[10] In 2015, Leonnig won the Pulitzer Prize for national reporting "for her smart, persistent coverage of the Secret Service, its security lapses and the ways in which the agency neglected its vital task: the protection of the President of the United States.

"[11] The Washington Post received the 2014 Pulitzer Prize for Public Service for its coverage of the National Security Agency's expanded surveillance of everyday Americans based on former NSA contractor Edward Snowden's disclosures.

Leonnig was part of the reporting team whose six months of revelatory work exposed the government's secret collection of records for all Americans' phone calls and electronic communications.

[12] Also in 2014, Leonnig was a winner of the George Polk Award for investigative reporting, given by Long Island University, for her 2013 work uncovering a bribery scandal involving Virginia Gov.