Owing much to the art of his predecessor Ştefan Luchian, Tonitza was largely inspired by Impressionism, but he equally admired the discoveries made by Post-impressionist artists (their revolution in composition and Belle Époque splendor). Tonitza was notably critical of Nicolae Grigorescu, the major trend-setter in Romanian art, whose success over "peasant motifs", he stated, had "lured him to remain, for the rest of his life, in this rosy and light-hearted atmosphere". He equally objected to Grigorescu's influence over younger generations, which had led to "mannerism" and "nationalism" in choice of subjects, and the emergent urbane art ("where man shall represent only a decorative and amusing accessory").
Evidencing his "tormented life" and "fantasy-driven and bohemian lifestyle", Zambaccian wondered if these had not been the source of Tonitza's "ingenious art, full of chromatic joys that are nonetheless transited by melancholia". He drew a direct comparison between the artist's innovative presence in painting and George Bacovia's Symbolist poetry.
During his stay abroad, Nicolae Tonitza was influenced by the works of Rembrandt and Antonio da Correggio. An admirer of both Frans Masereel and Käthe Kollwitz, he also adapted Expressionist guidelines — ones especially present in his satirical drawings, but also manifested large works such as Coadă la pâine ("Queuing for Bread", 1920). According to Zambaccian, Tonitza stopped short of adopting clear Expressionist tenets ("Modigliani and Pascin favored contorting, while Tonitza does not stray away from nature and places an emphasis on feeling"); the two continued to oppose each other on the issue of Henri Matisse's style (admired by Zambaccian, by hotly contested by Tonitza). A more distant but no less direct influence was the graphic art of Honoré Daumier, which Tonitza had studied.
The early art produced by these influences was described in Sburătorul by Şirato, Tonitza's friend, as "paintings which are in fact drawings with a light resonance of intellectualism"; during the period, Rampa magazine hailed the painter as "A priest of humanitarian ideas, of ideas demanding the attention of present-day world leaders, with a more and more clear and audacious tone".
Most of his works are serene in tones, in contrast with those expressing Tonitza's involvement in social issues. They proposed a classical aesthetical ideal, viewing art as a treasurer of spiritual values. This message is most obvious in his Northern Dobruja landscapes, his still life studies, the portraits of clowns (celebrated for their way of sublimating the comic and grotesque elements in masks and makeup, in order to reveal a sad humanity), young women and children. The so-called "Tonitza eyes", both point-shaped and expressive, are a characteristic trait in his children portraits. In contrast with their appreciation for these pieces, Zambaccian and other members of Grupul celor patru expostulated the Balchik landscapes: Zambaccian remarked that his were "more like arabesques in colored tones, [...] at a time when Şirato evolved upward toward a nuanced painting of a beautiful representativeness in a luminous space".