He worked as a political assistant in the Ministry of Employment from 1976 to 1978, where Per Ahlmark was minister and simultaneously the Liberal People's Party's leader.

In 1983 Westerberg was the head of office of a foundation promoting free market solutions, where he worked together with Carl Bildt of the Moderate Party, and wrote for the business weekly Affärsvärlden.

In October 1990 Westerberg and the Moderate Party leader Carl Bildt published an article where they declared that they wanted to govern together after next year's election.

Westerberg had declared ahead of the election that he did not want to be part of a government that needed New Democracy's support, and on the election night (when he had seen his party reduced to 9.1 per cent support) he refused to stay in the TV studio when New Democracy's Ian Wachtmeister and Bert Karlsson entered it.

Instead, the four-party centre-right Bildt Cabinet was formed, also including the Centre Party and the Christian Democrats, which needed the passive support of New Democracy in parliament.