He has held important positions with the Intesa Sanpaolo Banking Group, where he worked from 1981 to 2013. In particular, after having held office as General Manager of Banco di Napoli S.p.A. (2009–2013), covering the position of Regional Director for Campania, Basilicata, Calabria and Puglia, in July 2012 he took over the management of IntesaSanPaolo Group’s domestic commercial banking division (with direct responsibility for more than 20 banks pertaining to the group network and approximately 47,000 employees) and in December 2012 he was appointed General Manager of Intesa Sanpaolo Group, retaining his responsibility for the domestic commercial banking division as well as taking on the role of interim General Manager of Banco di Napoli. After having resigned from Intesa Sanpaolo Group in June 2013, Giuseppe Castagna was appointed as chairman of Italian Private Debt Initiative of Muzinich & Co. (an institutional asset manager specialising in corporate credit). He was Board Member in banks and other financial intermediaries (including: Société Européenne de Banque S.A. Luxembourg, Mediofactoring S.p.A., Leasint S.p.A., Banco di Napoli S.p.A., IMI Fondi Chiusi SGR S.p.A., Intesa Sanpaolo Private banking S.p.A. and Bank of Alexandria).
From 2014 to December 2016 he was General Manager and CEO at Banca Popolare di Milano and since January 2017 he has been CEO of the new Group Banco BPM.
In 2020, Giuseppe Castagna is awarded the title of Cavaliere del Lavoro by the President of the Italian Republic, Sergio Mattarella