Director of the US Secret Service Randolph Alles speaks at the Justice Department past year. Alles will remain in his role until then.
Alles' departure comes on the heels of Nielsen's resignation and, according to a source, has nothing to do with the recent Mar-a-Lago incident, in which a woman was arrested at the president's Florida property after allegedly carrying two Chinese passports and malware.
Speaking anonymously, three administration officials told the Associated Press Alles' departure stemmed from a personality conflict within the agency.
Trump recently empowered adviser and immigration hardliner Stephen Miller to take charge of Team Trump's border policies "and he's executing his plan" by dismantling Homeland Security's leadership, the official said.
Last week, Mr Trump said he "could not be happier" with the Secret Service.
The removal of Alles marked the latest in a long line of firings and resignations of senior officials in the Trump administration, a list that has included the secretaries of state, defense, homeland security, interior, veterans affairs and health and human services, the attorney general and numerous senior White House aides. Alles reported to Nielsen.
But administration officials told Fox News on Sunday that Nielsen's background in cybersecurity made her a poor fit to handle border issues, while McAleenan best fits Trump's requirement of being the "toughest cop" on the frontier.
CNN reporting a full purge may be coming at DHS: "United States Citizenship and Immigration Services director Francis Cissna and Office of the General Counsel's John Mitnick are expected to be gone soon, and the White House is eyeing others to be removed".
The Secret Service does not determine who is invited or welcome at Mar-a-Lago; this is the responsibility of the host entity.
Alles - a 35-year veteran of the Marines - had also had leadership posts in Customs and Border Protection and led the agency's Air and Marine Operations.