The columned, 47-foot-tall domed structure just west of the World War II Memorial, was reopened Thursday, The Washington Post reported.
Officials apparently wanted the memorial open in time for Veterans Day (Friday).
The D.C. Memorial honors the 20,000 Washington residents who served in World War I, including 499 who died in the war.
The $3.6 million restoration was ordered after the memorial fell under extreme disrepair, officials said. Its marble cracked and became discolored and its landscape was overgrown with shrubs and trees.
"For too many years it was a lonely orphan on this part of the Mall," Edwin L. Fountain, vice president of the World War I Memorial Foundation, told an audience at a reopening ceremony. "It was a forgotten memorial to a forgotten war."
During its restoration, the memorial's stone was cleaned and damage fixed. The landscape was cleaned up and fieldstone walkways were restored.
"Look at the landscape and how open it feels, how shiny and bright it looks," said Caroline L. Cunningham, president of the Trust for the National Mall, which made money for the restoration available.
"It's absolutely wonderful," said National Mall and Memorial Parks Superintendent Bob Vogel.