After working as an environmental organizer in Michigan, Ohio, and Pennsylvania, Popkin pursued various positions in community development in Philadelphia.

In 1999, he began working on a book of reportage, Song of the City: An Intimate History of the American Urban Landscape, published by Four Walls Eight Windows in 2002.

In 2007, he was named writer in residence of Philadelphia University, where Song of the City was taught to all freshmen as part of the First Year Experience program.

In 2011, Popkin co-founded the Hidden City Daily,[2] a web magazine covering Philadelphia's past, present, and future.

After the election of Donald J. Trump in November 2016, Popkin co-organized (along with poet Alicia Askenase and novelist Stephanie Feldman) Writers Resist Philadelphia, a day of literary protest January 15, 2017.

[28] In 2018, Popkin wrote the documentary film Sisters in Freedom,[29] which was produced by History Making Productions.