Answer by taking a quote from the following article:

Pieter Cornelis "Piet" Mondriaan, after 1906 Mondrian (; Dutch: ['pit 'mondrija:n], later ['mondrijan]; 7 March 1872 - 1 February 1944), was a Dutch painter and theoretician who is regarded as one of the greatest artists of the 20th century. He is known for being one of the pioneers of 20th century abstract art, as he changed his artistic direction from figurative painting to an increasingly abstract style, until he reached a point where his artistic vocabulary was reduced to simple geometric elements. Mondrian's art was highly utopian and was concerned with a search for universal values and aesthetics. He proclaimed in 1914: Art is higher than reality and has no direct relation to reality.

Unlike the Cubists, Mondrian still attempted to reconcile his painting with his spiritual pursuits, and in 1913 he began to fuse his art and his theosophical studies into a theory that signaled his final break from representational painting. While Mondrian was visiting the Netherlands in 1914, World War I began, forcing him to remain in there for the duration of the conflict. During this period, he stayed at the Laren artists' colony, where he met Bart van der Leck and Theo van Doesburg, who were both undergoing their own personal journeys toward abstraction. Van der Leck's use of only primary colors in his art greatly influenced Mondrian. After a meeting with Van der Leck in 1916, Mondrian wrote, "My technique which was more or less Cubist, and therefore more or less pictorial, came under the influence of his precise method." With Van Doesburg, Mondrian founded De Stijl (The Style), a journal of the De Stijl Group, in which he first published essays defining his theory, which he called neoplasticism.  Mondrian published "De Nieuwe Beelding in de schilderkunst" ("The New Plastic in Painting") in twelve installments during 1917 and 1918. This was his first major attempt to express his artistic theory in writing. Mondrian's best and most-often quoted expression of this theory, however, comes from a letter he wrote to H. P. Bremmer in 1914:  I construct lines and color combinations on a flat surface, in order to express general beauty with the utmost awareness. Nature (or, that which I see) inspires me, puts me, as with any painter, in an emotional state so that an urge comes about to make something, but I want to come as close as possible to the truth and abstract everything from that, until I reach the foundation (still just an external foundation!) of things...  I believe it is possible that, through horizontal and vertical lines constructed with awareness, but not with calculation, led by high intuition, and brought to harmony and rhythm, these basic forms of beauty, supplemented if necessary by other direct lines or curves, can become a work of art, as strong as it is true.

What was Mondrian works were influenced by?
During this period, he stayed at the Laren artists' colony, where he met Bart van der Leck and Theo van Doesburg,