The Secret Service is an agency that provides protection for president Trump and other top U.S. officials.
Trump instructed his Acting Chief of Staff, Mick Mulvaney, to fire Secret Service Director, Randolph Alles, CNN reported, citing multiple administration officials.
The White House, Department of Homeland Security and Secret Service did not immediately respond to requests for comment from Reuters.
One official described the firings to CNN as a near-systematic purge at the Department of Homeland Security.
The Secret Service director reports to the director of Homeland Security.
Nielsen, who oversaw Trumps bitterly contested immigration policies during her tumultuous 16-month tenure, resigned on Sunday amid a surge in the number of migrants at the border with Mexico.
Trump, seeking re-election in 2020, has taken a hard line toward immigration since becoming president in January 2017, and the issue may be a central theme in next years election.