In Palmerston North he was a dentist, and a local body politician. He was dux at Greymouth District High School. Tennent was a councillor for Palmerston North City Council from 1933 to 1941.
He was Mayor of Palmerston North from 1956 to 1959.
He was on the Board of Governors for Palmerston North Boys" High School, and in 1954 led the conservative opposition to the appointment of Guthrie Wilson to head either Palmerston North Boys" High School or Freyberg High School because of the frank and sexually explicit language in his novels. Tennent represented the Palmerston North electorate from 1949 to 1954, when he was defeated by Philip Skoglund.
He then represented the Manawatu electorate from 1957 to 1966, when he retired. He was Minister of Education in the Second National Government from 1960 to 1963.
He was appointed a Commander of the Order of the British Empire for services in politics and education in the 1973 New Year Honours.
Tennent died at his home in Palmerston North on 1 May 1976.