He has worked with many media, including painting, sculptures, ceramics and posters and has made designs for many companies, organisations and institutions, doing air planes, train stations, hospitals, architecture, monuments, stores, company profiles, handicrafts and everyday utensils.
Apart from his prolific poster art production, some of Arnoldi's best known single works are the logo and curtains designs of the Copenhagen Opera House from 2004 and London's National Police Memorial, co-designed with Peter Ridley from Foster + Partners.
Herning Museum of Contemporary Art in Denmark holds the largest permanent display of his work.
From 2008 to 2010, he was a member of Akademirådet (The Academy Council) at the Royal Danish Academy of Fine Arts and he was appointed chairman for Kunstrådet (The Art Council) in the Danish government administration for about a year in 2011.
Arnoldi is educated schoolteacher and worked as such for a short time before his interest in and engagement with painting, design and art became his primary occupation.