His father was active in the Victorian Farmers' Union and briefly served as president of the Deakin Shire Council.
[1] Tehan grew up in the rural locality of Timmering near Kyabram, where his father and uncle had a mixed farming property that included beef and dairy cattle, sheep, oats and wheat.
He subsequently left the public service and returned to Kyabram to practise as a lawyer, joining the firm of Morrison and Sawers.
He presented the case for separation to the Local Government Advisory Board and helped organise a referendum of ratepayers.
However, at the 1977 election he was beaten by Australian Democrats leader Don Chipp for the final Senate seat.
[1] In the Senate, Tehan spoke frequently on agricultural matters and was a "consistent advocate for the interests of the dairying, fruit-growing and canning industries of the Goulburn Valley region of Victoria".