He owned a brick manufacturing business, a wholesale grocery trade and developed the Philadelphia neighborhood now known as Rittenhouse Square.

Letters he sent from Washington, D.C., some of which have been preserved by the Historical Society of Pennsylvania, reflect a disgust with what Harper saw as an endemic of corruption by Andrew Jackson and his administration.

Following his retirement from Congress, Harper continued in the manufacture of brick, and branched out into real estate speculation and urban development.

He bought the north side of Philadelphia's then undeveloped Rittenhouse Square and built a fine house for himself at 1811 Walnut Street around 1840.

His mansion set a patrician residential tone for the square and he sold off the remaining lots at profit.