After World War II Roszak shifted from constructivist sculpture to abstract expressionist sculpture and was one of the first artists in his generation to turn his attention to welded metals as a medium for sculpture.  

"War can affect artistic perception. This was certainly the case with Theodore Roszak, whose work underwent a profound transition in the wake of World War II. Death, destruction, and the devastation of two Japanese cities revealed the darker side of technological progress. What previously had been a positivistic embrace of utopian systems was seriously questioned by the end of the war. Roszak's shattered faith in science and technology was replaced by a renewed faith in nature, in change and transformation, and in atavistic motifs that reaffirmed basic values. He wanted his work to ask questions (rather than posit definitive answers), to provoke, even rankle. He also wanted it to evoke archetypal themes and embody a life force that was destructive as well as constructive."

[Douglas Dreishpoon, "The Teresa and Alvin S. Lane Collection: Twentieth Century Sculpture and Sculptors' Works on Paper", Elvehjem Museum of Art University of Wisconsin-Madison, 1995, p.30]

In 1951, the Pierre Matisse Gallery started representing Theodore Roszak. He had 4 solo exhibitions with Pierre Matisse showing his sculptures, drawings, and lithographs until 1974. He was the only American artist represented by the gallery.