Kenney appointed Jim Engler to fill Slusser's job effective Aug. 10.
Engler, 33, has been Kenney's deputy mayor for policy and legislation. Prior to that, he was an aide to then-Councilman Kenney.
"I'm thrilled to select Jim Engler as my new chief of staff," Kenney said in a statement. "I have always tried hard to attract the smartest and most talented staff, and recognize the need to empower them and listen to them. Jim Engler is a perfect example of this. Over the past half-decade, I've come to depend on his advice and judgment on the most complex issues facing our city."
Kenney's office said Engler helped pass some of his top legislative initiatives, including the soda tax and infrastructure program known as "Rebuild," while serving as the liaison between the mayor's office and City Council. He was also one of Kenney's top advisers.
Engler was born and raised in the same South Philadelphia neighborhood as Kenney and graduated from Kenney's alma mater, St. Joe's Prep, though a few decades later.
"I have an aunt who went to grade school with him. So our families have known each other for a while," Engler said.
Engler said he watched Kenney's rising career as a councilman with admiration.
"I remember him standing up for immigrants back in the 2000s even at a time when it wasn't popular for his base. … That really impressed me," he said.
After Engler received his law degree from Temple University, he went to work for Councilman Kenney as his legislative director. Engler helped draft bills to decriminalize possession of small amounts of marijuana and to reform the city's construction-safety regulations. When Kenney decided to run for mayor, Engler followed and became policy director for Kenney's mayoral campaign in 2015.
"It was a whirlwind," he said of the campaign. "You get few opportunities to work for someone who you really believe in, who has his heart in the right place all the time."
Engler and Slusser worked together during the campaign, which made for a smooth transition to working in the mayor's office, Engler said.
"There was no divide," Engler said of him and his colleagues from Councilman Kenney's office, and the campaign professionals — such as Slusser — who also joined the administration as cabinet members.
Slusser, 35, was seen as one of the leading progressive voices in the Kenney administration. Mustafa Rashed, a local political consultant who lobbied for the city's soda tax, said that "Jane's voice has helped to shape every major policy issue and initiative for Philadelphia" under Kenney.
Slusser said that Engler was also part of the discussions and decisions on progressive issues.
"When I'm building an internal coalition for a progressive move, Engler is always the one there. … He's been at the forefront of all our major policy decisions," Slusser said.
As for Slusser's next move, she wouldn't give much detail on what she is up to, other than to say: "I will be moving into a new role focused on mobilizing voters in advance of this November's midterm elections."
She added that she's been proud to work for the city and stand up to President Trump. The Kenney administration recently won a legal challenge against the U.S. Department of Justice to keep its "sanctuary city" policies without losing federal grant money. Just last week, Kenney decided to not renew its data-sharing contract with U.S. Immigration and Customs Enforcement.