BETHLEHEM, Pa. — Ellen Gaugler remembers driving her father to the Bethlehem Steel mill, where he spent his working years hauling beams off the assembly line and onto rail cars.

When the Pennsylvania plant shut down about two decades ago, Ms. Gaugler thought it was the last time she or anyone in Bethlehem would come to its gates to find a job that paid a decent wage for a physical day of work.

But she saw an ad in the paper last year for a position at a local warehouse that changed her mind. She’d never heard of Zulily, the online retailer doing the hiring, but she knew the address: It was on the old mill site, steps from where her father worked.