[1] Withnall was appointed to the Northern Territory Legislative Council in 1960 in his capacity as Crown Law Officer.

[3][4] As a Member of the Legislative Council, Withnall made a name for himself as a fierce advocate of self-determination, and took part in the campaign which ultimately led to the creation of the Assembly in 1974.

However, he was to only serve one term in the Assembly, and was replaced by the Country Liberal Party's Tom Harris.

Shane Stone, in his introductory speech upon his election to the Port Darwin electorate in 1990, referred to Withnall as a "fearless and uncompromising advocate of self-government and statehood for the Territory".

In 1994, then-Chief Minister Marshall Perron also praised Withnall's role in the Territory's achievement of self-governance.