He initially trained as an artist and at the age of seventeen won a scholarship to the Art Students League in New York City to study fashion drawing.
He transferred to New York University's Institute of Fine Arts for his master's degree, which he earned in 1957 with the thesis "The Statues in Watteau's Paintings".
During the thirty-five years that Munhall served as Curator at The Frick Collection, the institution produced the first multi-volume catalog of its holdings and began mounting special exhibitions on a regular basis.
Munhall also contributed to a series of major acquisitions, including paintings by Gentile da Fabriano, Hans Memling, and François-Hubert Drouais.
Munhall organized or contributed to twenty-eight exhibitions mounted at The Frick Collection and other institutions, including: "Jean-Baptiste Greuze, 1725-1805" (1976-1977),[5] the first exhibition to focus on that artist;[4] "Severo Calzetta called Severo da Ravenna" (1978); "Jean-Antoine Houdon: Eight Portrait Busts" (1981); "Ingres and the Comtesse d'Haussonville" (1985-1986); "François-Marius Granet: Watercolors from the Musée Granet at Aix-en-Provence" (1988); "Nicolas Lancret" (1991); "The Butterfly and the Bat: Whistler and Montesquiou" (1995-1996);[6] "Sir John Soane: Collector and Connoisseur" (1996); and "Victorian Fairy Painting" (1998).