Manigault Newman, one of Trump's most high-profile African-American supporters, will leave one year to the day after Trump took office, Sanders said.
"Omarosa Manigault Newman resigned yesterday to pursue other opportunities," Sanders said. "Her departure will not be effective until January 20, 2018. We wish her the best in future endeavors and are grateful for her service."
CNN has reached out to Manigault Newman but has not heard back.
"Thank you Omarosa for your service! I wish you continued success," President Donald Trump tweeted Wednesday night.
White House officials have long expected that some Trump aides would leave the White House in January and Manigault Newman is the latest example that an exodus from the White House could happen. White House aides announced earlier this month that Dina Powell, Trump's deputy national security adviser, plans to leave after the President's first year in office.
It is traditional for administrations to experience a spate of departures after the first year in office, but Trump's year has been particularly bruising, leaving some officials preparing for more departures than usual.
Manigault Newman's departure, according to two sources close to Trump, has been a long time coming. She joined the Trump White House as the top communications official at the White House Office of Public Liaison, but when chief of staff John Kelly took over in July, her role began to feel ill-defined, the sources said.
It was her loyalty and friendship with Trump -- something that was fostered during the 2016 campaign -- that kept her there for longer than anyone expected, one source added.
Manigault Newman last traveled with Trump to the Mississippi Civil Rights Museum in Jackson on Saturday. But according to a White House official, the Manigault Newman has had no access to Trump for months and is considered to be a non-stop problem in the West Wing.
Both Kelly and former chief of staff Reince Priebus wanted to let Manigault Newman go, a former White House official told CNN.
"People have long been unsure what she did at the White House," the former official said, adding that people inside the White House had been openly wondering "what value she brought" to the operation.
"Many of her colleagues are elated by today's news," the former official added.
Manigault Newman, who was married earlier this year, was one of Trump's most outspoken supporters during the campaign.
In an interview with PBS "Frontline," Manigault Newman said that when Trump wins, "Every critic, every detractor, will have to bow down to President Trump."
Much of Manigault Newman's work centered around being a high-profile Trump surrogate with African-American voters. That role, however, was often contentious and her appearance at the National Association of Black Journalists' annual convention in New Orleans earlier this year spiraled into a shouting match
about Trump's views on police brutality.