"As some of you are aware, yesterday I offered my resignation to Secretary McAleenan, effective Friday, July 5," Sanders wrote in a message to staff. "Although I will leave it to you to determine whether I was successful, I can unequivocally say that helping support the amazing men and women of CBP has been the most fulfilling and satisfying opportunity of my career."
A former technology executive, Sanders had a reputation as a low-key and nonpartisan figure at CBP. He served under McAleenan as CBP chief operating officer and before that worked at the Transportation Security Administration.
Another DHS official said personnel changes were also underway at US Immigration and Customs Enforcement (ICE), whose acting director Mark Morgan was meeting today with senior CBP leaders.
The meetings fuelled speculation that Sanders was removed to clear a path for Morgan to take over at CBP.
Morgan worked at the FBI under former director James Comey, and was brought in to CBP in 2014 to lead an overhaul of Border Patrol use-of-force policies. The effort was successful, but it chafed at senior CBP officials, who viewed him as an outsider who had not paid his dues by working as a rank-and-file border agent.
Morgan ascended to be head of the US Border Patrol at the end of President Barack Obama's second term, but he was removed from his job when Trump took office.
He worked his way back into Trump's good graces through a series of appearances praising the President on Fox News. Trump named Morgan to the top job at ICE last month, saying he wanted to go in a "tougher direction."
Morgan had never worked at ICE, though, and made his preference for the top CBP job clear to colleagues. A push by the White House to have him take over leadership of CBP - instead of ICE - was blocked last month by McAleenan.