As a lyricist, Maschwitz wrote the screenplays of several successful films in the 1930s and 1940s, but he is perhaps best remembered today for his lyrics to 1940s popular songs such as "A Nightingale Sang in Berkeley Square" (music by Manning Sherwin and Jack Strachey) and "These Foolish Things" (music by Jack Strachey). Romantically linked to the Chinese-American actress Anna May Wong while working in Hollywood, the lyrics of "These Foolish Things" are evocative of Maschwitz' longing for her after they parted and he returned to England.
Maschwitz started his stage acting career in the early 1920s and joined the BBC in 1926. His first radio show was In Town Tonight and his first television show was The Black and White Minstrel Show. In 1939, he went to Hollywood under contract to MGM, where he worked on Goodbye, Mr. Chips (for which he shared an Academy Award nomination) and Queen of Song, among other successful films.
From August 1939, he was a postal censor in Liverpool. From November 1939, he served with the Secret Intelligence Service (SIS)/MI-6 D Section (sabotage). In 1940, he briefly worked to establish a resistance organization in Beverley, Yorkshire, and for Army Welfare in London; and then was assigned to the Special Operations Executive (SOE). In 1940 he was commissioned into the Intelligence Corps. He was then sent to New York City to work for the British Security Coordination (BSC). In 1942, he returned to London. In 1942, he briefly supervised radio programs for the troops. He then transferred to the Political Warfare Executive (PWE). He ended the war as chief broadcasting officer with the 21st Army Group. He left the army as a Lieutenant-Colonel.
In 1958, at the start of the BBC/ITV ratings wars, he rejoined the BBC as Head of Television Light Entertainment. About the job he said, "I don't think the BBC is a cultural organisation. We've got to please the people. The job of a man putting on a show is to get an audience." Maschwitz left to join the rival ITV in 1963.
During the course of his varied entertainment career, Maschwitz also adapted French comedies such as Thirteen For Dinner; wrote the book and lyrics for numerous musicals, amongst them Balalaika, Summer Song, The Ghost Train and Zip Goes a Million; and created Café Collette. He also edited the Radio Times, and even turned his hand to the detective novel: Death at Broadcasting House, co-written with Val Gielgud and published in 1931, revolves around a radio play disrupted by the murder of one of the cast.
He was created an Officer of the British Empire (OBE) in 1936.