After numerous reports that Bannon was leaving, the White House released a statement on Friday saying, "White House Chief of Staff John Kelly and Steve Bannon have mutually agreed today would be Steve's last day."
The controversial advisor, who holds populist nationalist ideas, helped propel Trump to office. However, tension has been festering between Bannon and other top advisors to Trump. On Wednesday, Reuters reported that discord between Bannon and national security advisor H.R. McMaster is destabilizing Trump's team.
Bannon is the latest official to leave the administration. Press secretary Sean Spicer, chief of staff Reince Priebus and Anthony Scaramucci, who served a short stint as communications director, are among the recent high-profile departures.
Bethune noted that Trump chooses his staff, but then he later publicly humiliates some of them.
"Who's going to want to work in that environment? You've got to build loyalty through your employees and to your constituencies," Bethune said.
"He's a poor business leader and poor country leader."
The White House did not immediately respond to a request for comment.
— CNBC's Jacob Pramuk and Fred Imbert contributed to this report.