The impressionistic and realistic work that marked his early career morphed into the most simplistic abstraction of line and color. When I explored his paintings, I was interested to see the various self portraits he completed over time. His 1900 self-portrait is very impressionistic, flat, and direct. It is a far cry from his 1942 pencil representation.
“In 1911, Mondrian attended the exhibition of Georges Braque. The work of the French Cubist impressed the Dutch painter greatly, as it paralleled much of what he had been experimenting with on his own, and Mondrian resolved to visit Paris, fascinated by the artistic innovations being introduced there.” (http://www.abcgallery.com/M/mondrian/mondrianbio.html) Where Mondrian deviated from Cubists like Picasso and Braque was in his application of the style to landscapes rather than people or still life images.
When you look at his tree paintings of 1912, you can almost watch the transition in real time. The Grey Tree is still recognizable as a tree, although stylized. Trees in Blossom has been simplified, but still symmetrical and tree-like. Composition Trees II has almost lost all of its nature as a rendering of a recognizable tree.
The painting Composition with Yellow Lines seems to be the penultimate simplification.
“Through all this, Mondrian advanced with his theory. As during all his periods of experimentation -- and probably as a consequence of frequent travel -- his output dropped, but each piece was executed with a lot of thought and deliberation. New York, New York (1941-42) features a larger number of colored lines, as the artist sought to break up the established structure of his work. By New York City I (1942), black lines are completely gone. In Broadway Boogie-Woogie (1942-43), the colored lines are broken up with patches, completely defying the definition of anything that could be called drawing.” (Olga website)