Secret Service officials have been caught by surprise with the news and are only finding out through CNN, according to the source.
In a statement, White House Press Secretary Sarah Sanders said Alles "has done a great job at the agency" and would be leaving shortly to be replaced by James M. Murray, a career Secret Service member, who will start in May.
Still, it comes amid a spate of turnover across DHS that began last week when Trump withdrew his Immigration and Customs Enforcement director's nomination to stay on permanently.
Nielsen, who oversaw Trump's bitterly contested immigration policies during her tumultuous 16-month tenure, resigned amid a surge in the number of migrants at the border with Mexico.
Just five days ago, Trump said he "could not be happier with Secret Service" following the Mar-a-Lago incident.
Alles has been asked to leave on Monday, a number of U.S. mainstream media outletsreported citing anonymous administration officials.
"There is a near-systematic purge happening at the nation's second-largest national security agency", a senior administration official told CNN.
A source told CNN that Alles' removal is not connected to recent scrutiny of the Secret Service after a Chinese woman illegally entered the president's Mar-a-Lago club in Florida carrying Chinese passports and a flash drive containing malware.
Citizenship and Immigration Services Director Francis Cissna and Office of the General Counsel's John Mitnick are two other officials expected soon to leave the department.
(Saul Loeb/AFP/Getty Images, FILE) From left, Secret Service Director Randolph Alles, first lady Melania Trump, President Donald Trump and an unnamed person, look at vehicles at the United States Secret Service James J. Rowley training facility in Beltsville, Md., Oct. 13, 2017.
Prior to running the Secret Service, Alles had served as the acting deputy commissioner of U.S. Customs and Border Protection; he also served in Operation Iraqi Freedom, commanding Marine Aircraft Group 11 Third Marine Aircraft Wing and retired as a major general in the United States Marine Corps after serving in the Marines for 35 years.
The FBI has identified him as the person who claimed to be Timmothy Pitzen , who disappeared in IL in 2011 at the age of six. A day of false hope has given way to questions about why a man would claim to be an IL boy who disappeared eight years ago.
Harsh criminal and anti-LGBT laws went into effect on Wednesday in Brunei amid outcry from human rights groups and celebrities. Foreign Minister Marise Payne said Australia had raised concerns with the Brunei government over the new penal code .
These products won't decline almost as much if interest rates reverse course and they still offer relatively attractive yields. To sum up, Capital Economics isn't expecting recession, but it is expecting slower USA and global growth.
Smollett had faced 16 felony counts alleging he lied to police about a racial and homophobic attack against him in January. The decision to drop charges has led to harsh criticism from the Chicago mayor , police and outside district attorneys .
The star is also the father of a 28-year-old son, Weston from his previous relationship with actress Christina Fulton. He was married to fellow Oscar-winner (and Bringing Out the Dead co-star) Patricia Arquette from 1995 to 2001.
Trump has said association health plans would let Americans "escape some of Obamacare's most burdensome mandates". He says Republicans are working on a replacement plan, but Congressman Peter Welch says that simply isn't true.
There are a great (number of) foreign nationals in our country who are here legally, who respect our laws and respect our people. Meaning, let's have a sober immigration policy informed by the interests of South Africans. "There is no straight line".
Jeff Bezos, wife agree on divorce terms
The ex-wife of Amazon founder Jeff Bezos has revealed some of the details of their divorce . "She is resourceful and brilliant". MacKenzie Bezos also said she will give up her interests in the Washington Post , a frequent target of criticism from U.S.
The Avengers Assemble for Round 2 in a New Endgame Trailer
This kind of ticket demand isn't unexpected - Fandango has created virtual "waiting rooms" for big releases in the past. These two haven't seen each other since the events of Captain America: Civil War when Steve chose Bucky over Tony.
Social media reacts to Neil Warnock post Chelsea defeat
"In pre-season after four or five training sessions, I realised that Callum was a player with very great potential", Sarri said . The Blues scored an equaliser - which replays showed was offside - before Ruben Loftus-Cheek headed home in the 90th minute.
Cities around the world flip the switch to mark Earth Hour
As the theme says, it's a call for all of us to change the way we live, the way we consume energy, to get into a saving mode. Since then, the movement has grown globally and is now the world's largest grassroots movement for the environment.
President threatens to close border with Mexico next week
Overall, attempts to get across the border into the United States illegally are down substantially from a decade or more ago. Mexican Foreign Minister Marcelo Ebrard shrugged off Trump's threat in a Twitter post that emphasised the countries' ties.