Iselin completed an apprenticeship before travelling first to Cologne and then Copenhagen in 1740, In Copenhagen, Iselin initially worked for Fabritius & Wever, a trading house owned by Michael Fabritius and Johan Frederik Wæver.

In 1749 he established his own trading house under the name Reinhard Iselin & Co. which soon grew to be one of the most important in Copenhagen.

In the same year, he obtained a royal license to establish a calico textile manufactury at the Christiansholm estate.

He owned Rosendal and Rosenvænget at Copenhagen as well as a country house (now Gramlille) in Kongens Lyngby.

Iselin married his former employer Michael Fabritius' daughter Anna Elisabeth on 2 February 1772 in the German Reformed Church in Copenhagen.

In accordance with his wishes, she established two estates (stamhuse) for their daughters in 1781, Iselingen and Rosenfeldt (named after the family's home town, Rosenfeld in Württemberg).