Trained in academic art, initially an Impressionist, he dabbled in various modern styles in the years before World War I. Theodorescu-Sion's palette was interchangeably post-Impressionist, Divisionist, Realist, Symbolist, Synthetist, Fauve or Cubist, but his creation had one major ideological focus: depicting peasant life in its natural setting.

In time, Sion contributed to the generational goal of creating a specifically Romanian modern art, located at the intersection of folk tradition, primitivist tendencies borrowed from the West, and 20th-century agrarian politics.

[3] On both sides, his family had origins in Transylvania's Apuseni Mountains and the Breadfield, regions at the time still part of Austria-Hungary; by popular account, some were Moți, that is to say ethnic Romanian herders with a distinctly rustic lifestyle.

[19] Building on the conclusions of other researchers, such as Theodor Enescu, literary historian Paul Cernat sees in this movement, called "anti-academic post-impressionism", Romania's first departure from picturesque salon art, as well as a Romanian version of the Armory Show phenomenon.

[6] It was here that he first met art collector and mecena Krikor Zambaccian,[6][8] who would purchase an exhibit a sizable portion of his later canvasses, including Moara din Balcic ("The Balchik Mill").

[8] Around 1914, Sion had distanced entirely himself from Symbolism and the decorative lines of Art Nouveau, advising his young pupil Lola Schmierer Roth to do the same—they both relied on a proto-Cubist composition into solid shapes, for which the model was Cézanne.

[6] In 1918, Sion joined them as they broke with Tinerimea, creating the new artistic forum Arta Română ("Romanian Art")—Ressu, Nicolae Tonitza, Ștefan Dimitrescu and Oscar Han were among the other main affiliates.

[28] He still flirted with socialism, and, as noted by journalist Tudor Teodorescu-Braniște, helped out in the 1920 funeral ceremony of Marxist theorist Constantin Dobrogeanu-Gherea: "The great old man [...] was being laid down on a bier that a group of socialist painters, headed by Teodorescu Sion, had previously wrapped in red fabric.

[30] During the interwar period, in addition to Tinerimea and Arta Română salons, Theodorescu-Sion exhibited his work at the Atheneum, the Universul newspaper art show, Dalles Gallery and various other venues.

[2] With Ressu and Arthur Verona, Sion was also co-founder, in 1921, of Romania's first artists' trade union (Sindicatul Artelor Frumoase), which militated for basic social security but also had a political and (according to art expert Vasile Radu) "utopian" agenda.

Welcomed there by art columnist Oscar Walter Cisek,[34] and later by editor in chief Nichifor Crainic,[35][36] he provided illustrations to 1923's Satul meu ("My Village"), by Gândirist poet laureate Ion Pillat.

[6] Another former Symbolist, Tudor Arghezi, welcomed the change in style, writing: "Whosoever remembers Theodorescu-Sion's angles, points, squares, circles, semicircles and spheres will have thanked him for handing in his cap and taking on a better suited hat, and for parting with the confabulations of a very reduced Symbolism".

[41] Of all the paintings he presented for the public during the Ileana Gallery Art Show in 1925, the vast majority were landscapes of the mountains, or compositions with shepherds and mountain-folk such as La isvorul Troiței ("At the Troița [Trinity] Spring"), alternating with new Balcic seascapes.

He sat at the same table with some of the modernist and neo-traditionalist writers (Camil Baltazar, Liviu Rebreanu, Vasile Voiculescu, Ilarie Voronca), and, story goes, was once caught up in a cake fight with the satirist and prankster Păstorel Teodoreanu.

Writing for the Transylvanian readership of Luceafărul, in May 1909, George Murnu contended: "Teodorescu Sion is a new talent that ought to work hard before reaching the profundity of observation which makes one an artist.

"[56] As noted in 1913 by Luceafărul art columnist G. Duma, Sion took his painting "science" from French academies and "the grand masters", techniques from Synthetism in his religious work, and modern decorative elements in his landscapes.

[57] Duma describes Sion's 1913 paintings, especially Crai nou ("New Moon") and the Crucifixion, as a spiritual journey, and concludes: "In vibrant colors, with well-ordered planes, always directing us to the art of the future, the conscious painter Theodorescu-Sion dreams of and puts into song that aria which leads to immortality.

Mariana Vida calls his early compositions "pathetically Symbolist",[59] but, according to Amelia Pavel, his visions of solitary trees fuse Art Nouveau, "with its twisted lineaments", with elements taken out of Expressionism.

[60] Pavel is explicitly contradicted by fellow Romanian scholar Dan Grigorescu: "the tree motif is probably closer to [Sion's] decorative-muralist vision of a hallmark monumentality, with its ethnicist implications".

[67] Another interwar critic, Aurel D. Broșteanu, writes that Sion (like Iser, but with more rustic influences) contributed to "the assimilation of a pictorial objectivity", and set it out against "amorphous and disorganized Impressionism.

[69] Amelia Pavel additionally writes that the mature Sion returned to painting trees, with Expressionist filtered through Derain's pictorial techniques and, more characteristically, with a growing interest in making others discover the rural landscape of Romania.

[70] According to the Meridiane authors, his "rhythmic sequencing of volumes" shows a mix of influences distilled from contemporary Constructivism and echoes of the Symbolist master Pierre Puvis de Chavannes.

Vianu proposes that Theodorescu-Sion successfully fabricated himself the mentality of mountain dweller, with broken horizons and human figures seen from up close, with a somber palette that suggests "the coolness and secrecy one finds in a forest canopy.

"[72] He notes: "one may document the nature of the man who created this painting style with his famous Self-portrait he exhibited [in 1925], where the protruding anatomy of his face, the unibrow, the one eye open and scanning, evoke in truth the very image of an ancestor from the mountains.

[75] As noted in 1931 by Broșteanu, there were three deep influences on Sion's still lifes and portraits: the antique ceramic art of Romania, alongside Derain's pictorial vision, and, "unmistakably" so, the post-Impressionist canvasses of Ștefan Luchian.

"[39] This, Cisek argues, was the revelation of a true artistic expression of the Romanian soul, opposed to the idyllic canvasses of Impressionist master Nicolae Grigorescu, but akin to the Poporanist prose of Calistrat Hogaș.

[49] Although he constructs a similar argument, the religious commentator Crainic finds that Sion was less secure as a Christian artist, even though, he notes, Gândirea itself encouraged him to paint modern Romanian icons.

[80] The more radical experimenters, including Sion's student Jacques Hérold, rejected tame modernism altogether, turning to Surrealism;[81] but young neo-traditionalists such as Elena Popea found in it a source of inspiration.

[51] Kessler additionally notes that there is a radical component to Sion's belief in the organic relationship of men and the soil, likening it to the main concepts of Romanian nationalism and traditionalism—from A. C. Cuza's antisemitism to Virgil Madgearu's agrarianism, and passing through Blaga's theory on folk architecture.

Writing in 2009, art critic Pavel Șușară denounced "an unacceptable disagreement" between Sion's status as a "first-class artist" and the low starting prices of his canvasses, as opposed to the "exorbitant sums" fetched by painters such as Sabin Bălașa.