After finishing High School in Karlsruhe in 1966, Benneter studied law, economics and political science at the Free University of Berlin from, passing examinations in 1971 and 1974. From 1971 to 1976 he worked as a lecturer at the School of Economics in Berlin. From 1975, he has worked as a lawyer.
Benneter joined the SPD in 1965. He was Vice-Chairman of the SPD youth wing from 1974 to 1977, becoming Chairman in 1977. Expelled from the SPD in 1977, he rejoined in 1983 and served as treasurer from 1990 to 1996 and thereafter deputy chairman of the Berlin SPD until 2000.
From 1999 to 2002, Benneter was a member of the Berlin House of Representatives. In 2002 he was elected to the Bundestag representing Berlin Steglitz-Zehlendorf. Since November 2005, Benneter is legal adviser of the SPD parliamentary group. He lost his constituency seat at the 2005 election but returned to the parliament as a list member for Berlin region. He contested the Steglitz-Zehlendorf seat again at the 2009 election. On 27 September, he lost his mandate, just missing the return to the Bundestag via the party list by a few percentage points, being at list position 5. He has since returned to practicing law.