With a style inspired by the Englishman National Gonella, he guested in 1938 in leading orchestras such as Hot Dogs and Funny Boys. Before his musical career, he was also one of the country"s leading cyclists. After trips to England (1938-1939) with Vic Lewis and George Shearing, he was a central part of Oslo"s swing-jazz milieu, where he led his own Rowland Greenberg Swing Band (1939-1941) with Arvid Gram Paulsen on sax, Lulle Kristoffersen on piano and Pete Brown on drums.
He also led his Rowland Greenberg Rytmeorkester (1940-1944), with Gordon Franklin on tenor sax, Arvid Gram Paulsen on alto sax, Robert Normann on guitar, Kjell Bjørnstad, Frank Hansen, Lyder Vengbo on trombone, Fred Lange-Nielsen on bass.
A record release (1942) was banned by the German regime, and he was jailed for breaching the Rytmeklubbforbundet by viewing jazz films (1943). After his release from Grini, he was also active in Sweden (with Cecil Aagaard, Thore Erling and Malte Johnson) and England (with Jimmie Woode and Sam Samson).
He also toured Norway with his own band (1948-1950), including presenting bebop to the country. Greenberg took part in the "All-Star Trumpets session" (Paris Jazz Festival, 1949) with Miles Davis, Bill Coleman, Jimmy McPartland and Aime Barelli.
Greenberg also played with Charlie Parker who joined Greenberg in his Sweden tour band (1950), along with Louis Armstrong (1952), and Down Beat, who gave Greenberg the first chart placing of his career.
During the 50s, he played extensively in the orchestras led by Egil Monn-Iversen, Leiv Flisnes and Terje Kjær. He led his own orchestras including Mikkel Flagstad on piano, Totti Bergh on saxsophone, Knut Young on bass, Ivar Wefring on piano, Bjørn Krokfoss on drums (until 1981), and played with Ben Webster (Moldejazz, 1969) and Teddy Wilson.