In 1917, Greenfield was elected to a seat on the Philadelphia Common Council and served until 1920. Originally a Republican, he switched parties with the advent of the New Deal in 1933 and remained a strong Democratic supporter until his death. He enjoyed a close relationship with many Presidents from Herbert Hoover to Lyndon Johnson. In 1951 he played a major role in the defeat of Philadelphia's long-entrenched Republican machine. As chairman of Philadelphia's City Planning Commission under Mayor Richardson Dilworth (1956–58), he laid the foundation for the development of Society Hill as a unique upper-middle-class enclave capable of luring suburbanites back to downtown. Because of his political activism, in 1948 Philadelphia hosted both the Republican and the Democratic party conventions.