As an undergraduate he was attracted to bibliographical work in Elizabethan authors and began to collect actively in the late 1950s while a lieutenant in the United States Army stationed in Germany.
His work for the Harold Hughes election campaign during this time landed him on the master list of Nixon political opponents.
He later practiced in New York City and then served in turn as Co-Chairman and CEO of Rothschild, North America, Senior Managing Director of Bear Stearns & Co., and Vice-Chairman of Investment Banking at SG Cowen Securities Corporation.
Pirie's extensive collection of rare books and manuscripts was auctioned by Sotheby's on December 2–4, 2015.
[1] In 2019 Nicolas Barker compiled The Pirie Library: A Short-Title Catalogue of the Collection of Robert S Pirie which was published by Bernard Quaritch, Ltd. [2]