The Daily News reported last year that Gillen, a Democrat, has had conversations about running for mayor in 2015, when Nutter's second term is up. City officials must resign to campaign for a new office.
When asked if her departure is a sign she's getting ready for a run, Gillen said, "You can fuel all that speculation but right now, I'm really just focused on paying the bills and getting back to economic development work.
"I'm not doing anything political right now. Later in the year, maybe I'll look at politics," she said. "This is really just straight up doing job-creation type policy."
Gillen ran for City Council in 1999 and for the state House of Representatives in 2004, losing both times.
Philadelphia has never had a female mayor, and all of the Democratic politicians seen as likely to run in 2015 are men.
They include City Controller Alan Butkovitz, state Sen. Anthony Hardy Williams, Councilman Jim Kenney, Councilman Bill Green, former Councilman Frank Rizzo Jr. and Council President Darrell Clarke.
Gillen, who has degrees from the University of Rochester, the University of Chicago and the University of Pennsylvania Law School, worked under Ed Rendell for seven years when he was mayor and for one year when he was governor.
She was political director for Nutter's 2007 mayoral campaign and led the Redevelopment Authority for the first three years of his administration.
In a statement, Nutter lauded Gillen as a "longtime friend and supporter" and a "strong leader with powerful analytical skill."
On Twitter: @SeanWalshDN