Mercia Bowser died at the age of 64 from the coronavirus after being treated for Covid-related pneumonia, the politician announced on Wednesday.
“Mercia was loved immensely and will be missed greatly, as she joins the legion of angels who have gone home too soon due to the pandemic,” the Democratic mayor said in a statement.
“I ask that you continue to keep those who have been lost or impacted by the pandemic and those who are working so hard to protect us from it in your thoughts and prayers.”
The mayor added that her eldest sibling “worked tirelessly for children, the elderly, and those with behavioural disorders until her retirement and beyond.”
The US has now seen more than 28.3 million cases of Covid and 503,000 people have died during the pandemic.
The country officially passed the 500,000 deaths mark on Monday, with Joe Biden making brief remarks at the White House to commemorate the losses.
“Today, we mark a truly grim, heartbreaking milestone,” said the president.
“500,071 dead. That’s more Americans who died in one year in this pandemic than in World War I, World War II and the Vietnam War combined.”
Washington DC has had 39,943 cases of Covid-19 with 1,001 deaths.