In 1937, his friend Louis Adamic helped him get the commission for what would end up being his most important work, a series of murals for St. Nicholas Church, a Croatian parish in the Pittsburgh suburb of Millvale, Pennsylvania.

During this time, he became convinced that the church was haunted by a ghostly, black-robed figure, which Adamic later wrote about in a piece for Harper's Magazine titled The Millvale Apparition.

[11] The subject matter of the murals includes a combination of traditional religious imagery and social themes related to the Croatian American experience, such as war, injustice, and exploitation of workers.

[4] Vanka acknowledged traditional church decoration practices dating back to the Byzantine Empire in his placement of Mary, Queen of Heaven, above the altar and depictions of the Ascension of Jesus and the Four Evangelists on the ceiling.

[4] Apart from the Millvale murals, the majority of Vanka's American work consisted of charcoal and pastel drawings, many of which depicted scenes and people he met during his travels, and oil paintings, which were predominantly landscapes and still lifes.

Vanka mostly stopped exhibiting his work after World War II,[14] although he did hold a small show consisting of "fruits, flowers, and allegorical landscapes" at the Charles Barzansky gallery in New York in 1957.

[9] In his later life, Vanka lived on a farm near Doylestown in Bucks County, Pennsylvania, and taught art appreciation at the National Agricultural College (now Delaware Valley University).

[16] This collection was initially displayed at Vanka's former summer home on the island of Korčula, but was later moved to the Strossmayer Gallery of Old Masters in Zagreb.

[21] On Easter Sunday 2012 the Pittsburgh-based band Action Camp released a short film setting one of their live performances of a musical suite against the Maxo Vanka murals that inspired it.