The company's $45.4 million loss stemmed from a write-down in the value of its Kaplan education division, offsetting a slim operating profit at its newspaper division and stronger results at its television operations.
Revenues for the fourth quarter rose one percent to $1.05 billion, and fell three percent for the year to $4.02 billion.
Its newspaper division remained under pressure: daily circulation at the Washington Post declined 8.6 percent in 2012 to an average 471,800, while Sunday circulation fell 6.2 percent to 687,200.
Newspaper publishing revenues fell six percent year-over-year in the quarter to $162 million and were down seven percent for the year.
Print advertising revenue at The Post slid 14 percent in 2012 to $228.2 million, with a 12 percent drop in the fourth quarter.
"The decline is largely due to reductions in general and retail advertising," a company statement said.
The group saw only modest gains in its online activities, which include the washingtonpost.com website and Slate, where revenues were up five percent for the year at $110.6 million.
Display online advertising revenue increased six percent in 2012 while online classified advertising revenue decreased one percent for the year.
The Post, one of the last major US newspapers to offer its content free of charge online, is expected to erect a metered paywall this year, where readers will have to pay for access beyond a minimum number of articles.
The Post is also considering the sale of its headquarters in the city's downtown district.
The past quarter's results were affected by a write-down of $111 million at Kaplan Test Preparation, reflecting the lower value of the business, which faces a more difficult economic environment.
Newspaper operations showed an operating profit of $2.6 million in the quarter, but a loss of $53.7 million for the full year.
Cable television delivered a quarterly operating profit of $43 million and broadcast television $62 million in operating earnings.