Having been brought to America in infancy by his parents, Nebel grew up New Jersey where he learned the rudiments of painting from a local woman. He worked for a time in his youth at a terra cotta factory in Perth Amboy, New Jersey, which specialized in architectural decorations. There, he learned how to model in clay and, thus, his interest in sculpture began. He continued studying architectural ornamentation in New York at the Mechanics Institute and took classes in drawing and sculpture at the Cooper Union and at the Art Students League. One of his instructors at the last place was James Earle Fraser with whom he maintained a life-long friendship and professional association. Fraser encouraged Nebel to compete for the Prix de Rome which he did, winning it in 1914. From that year until 1917, Nebel was at the American Academy in Rome on the fellowship. With the entry of the United States into World War I in 1918, he enlisted for service as a supply officer and interpreter for the Red Cross.
Nebel returned to the United States in 1920, bringing with him his wife, Maria Lucantoni who, as a professional model at the American Academy, had posed for the American artists such as Paul Manship. The Nebels settled at first in New York where Berthold worked as a studio assistant to Fraser; but later that year, he accepted the directorship of the School of Sculpture at the Carnegie Institute of Technology (now Carnegie-Mellon University) in Pittsburg. While living in that city, Nebel won a competition for a commission sponsored by the Bureau of Mines for the Mine Rescue Medal; about the same time, the Department of the Interior ordered the Congressional First Aid Medal from him. Undoubtedly inspired by Fraser, he also created a relief portrait of Theodore Roosevelt for the City County Building in Pittsburgh, and one of Arthur Anton Hamerscholog for the Carnegie Institute.
But Nebel missed the more exciting life of New York City and he and his family returned there in 1923. He established a studio on 8th Street. Among his public commissions from those years were ornamental sculptures for the Cunard Building in New York; a statue of Alabama's General Joseph Wheeler for Statuary Hall in the U.S. Capitol in Washington; another of General John Sedgwick for the State Capitol in Hartford, Connecticut; one of Alexander Brown for the entrance to Brown Brothers Bank in New York; bronze doors for the Museum of the American Indian and another set for the American Geographical Society Building, both in New York; and relief panels for the south facade of the Hispanic Society Building, also in New York. These last three projects were commissioned from Nebel by Archer Huntington whom the sculptor met in 1928 and who continued to give Nebel work and encouragement during the hard years of the depression. Another commission from Huntington was for the marble Nereid at Brookgreen Gardens, South Carolina.
In 1925, Nebel patented a machine used to enlarge sculptural models by using a scraping device as opposed to the pointing machines which sculptors had used since ancient times. During the 1940s and 1950s, he continued his interest in the medallic arts, creating medals for the Society of Medalists and the Medallic Art Company, among others. He was a member of the National Sculpture Society and the Architectural League of New York.
