After service in World War II and graduation from the University of Pennsylvania Law School, Hemphill embarked on a legal career before running for office.

He ran for mayor of Philadelphia in 1967 against the incumbent Democrat, James Tate, but was unsuccessful, and retired to a private law practice until his death in 1986.

He was born into a politically prominent family, members of which served for seven generations as Chester County Democratic Party chairmen.

[7] With the party united behind him, Hemphill defeated the incumbent Republican, Foster A. Dunlap, by thirteen percentage points and took office on January 6, 1958.

[8] Although he entered office as an ally of Dilworth, Hemphill's frequent investigations into the conduct of city officials soured the relationship, which grew into an open feud.

[10] Republicans demanded that a grand jury be convened to investigate further, but Judge Raymond Pace Alexander (who had served as a Democratic city councilman from 1952 to 1960) rejected their petition.

[14] Dilworth, who had previously called Hemphill a "pious phony," encouraged the Controller's investigations before resigning as mayor to run for governor.

[15] By 1962, Judge Joseph E. Gold agreed to convene a grand jury over the Frankford El scandal, and the investigation grew to encompass similar financial irregularities at the Philadelphia Gas Works; two city councilmen were indicted.

[18][19] He spent much his third term arguing with City Council over their attempt to borrow $20 million for the Gas Works in a procedure that Hemphill claimed lacked the necessary financial safeguards.

[24] In March, the Democratic City Committee, led by Francis R. Smith, endorsed Hemphill over the incumbent Tate.