He traveled widely in the United States and continued to explore throughout his life subjects that first captured his attention as an artist in the Thirties.

[3] Following his graduation from the Shanghai American School in 1927, Day came to the United States to study at the Art Students League of New York.

From 1933 until his military service in World War II, Day was represented by the Macbeth Gallery in New York – one of the major commercial venues of the time.

[6] Day's work gaining recognition included portrait studies, floral still lifes, urban scenes and rural landscapes of subjects in New York, Vermont, Georgia and the Carolina Lowcountry.

Founded by Lillian Wald, the Henry Street Settlement was in that period one of the pivotal centers of creative energy in New York.

In 1941, Day married the artist, Elizabeth Nottingham, who had also studied at the Art Students League of New York, and the two joined the faculty of Mary Baldwin College.

Equally at ease with landscape, portraits, still-life and figures, Horace has worked in the conviction that the age of great painting continues in our time.” [18] Very early in Day's career, a prominent critic of the time noted Day's exceptional sense for color and skill in handling color in both oil paintings and watercolor.

In his water-colors it is often softened to delicate nuances that testify to his sensitiveness – the envelope of this earth, the form that furnishes the foundation of his design.”[19] In commenting on Lefevre’s Quarry (1937), a painting of an abandoned marble quarry in Vermont that is now in the collection of the Fleming Museum, he observed, “Without the skillful use of fine color it would have been quite impossible to invest with aesthetic interest and value such a subject.

Here the thing itself – the quarry – is almost entirely hidden in a symphony of the greens of luxuriant foliage which awaken spiritual and emotional reactions entirely apart from and infinitely more important than any representation of such a forgotten hole in the ground.”[19] Day's “masterful command of color” and “ability to imbue his paintings with vibrant energy” was likewise marked in Day's later work.

[23] The spirit of the country was to be found, Day believed, “not simply in elegant antebellum homes, but also in the simple cabins found in the city and countryside.”[24] Day once wrote of the Lowcountry, “The landscape here is so luxuriant that it reminds me of South China.”[25] In fact, early landscapes from this period like Live Oak, Beaufort, SC (1938)[26] have been observed to bear a striking resemblance to View from Chang Chow City Wall, Fukien Province, China (1920), a painting Day executed while still living in China.

[9] In the Lowcountry as elsewhere, Day worked en plein air to capture directly the color and intensity of the scene.

As a result of that direct, emotional connection, Day's paintings have been acclaimed for revealing the essence of their place and the spirit of their people, and the sensitivity with which he recorded their character and the architecture of their homes, “whether simple wooden cabins set in a spreading field or pillared mansions overhung with majestic trees.”[27] In connection with an exhibition of his work at the Gibbes Museum in 2004, Day was acclaimed as having "captured the architecture, landscape and people of the area through charming, sensitive renderings in watercolor and oil, including Church, Edisto Island, SC (1960) and Live Oak Avenue, Edisto Island, SC(1960)."

Both paintings, it was remarked, demonstrate Day's "masterful command of color and the ability to imbue his works with vibrant energy and a solid sense of place.

In 1940, he applied to the Treasury Department's Section of Painting and Sculpture for a commission to do a mural for a United States Post Office.

Suspicion of Gas, K.P., and Reveille in Bivouac, were included in the exhibition, Art in the Armed Forces, at the National Gallery in London.

Country Club, an oil on paper, were included in the 1945 Annual Exhibition of Watercolors, Prints, Drawings and Sculpture at the Whitney Museum.

In connection with one recent exhibition[37] of these portraits that explored issues of style and identity as expressed by African-Americans in Alexandria, Virginia, during the 1970s, Adrienne Childs observed that "Horace Day demonstrated a high level of consciousness of the history and politics of representing blacks, as well as the social and psychological impact that the prevailing standards of beauty had on some people of color.