Westerberg was born in Nyköping and graduated from Stockholm School of Economics in 1974.

During the Speaker election in 2006 was, however, the MP who spent the second longest time in the Riksdag (Social Democrat Margareta Israelsson) acting Alderman because Westerberg in this election was himself disqualified.

[5] Westerberg was Minister for Industry and Trade in the Government of Carl Bildt from 1991 to 1994.

In 2003 he was elected as first of the three Vice Speakers of the Riksdag, and on 2 October 2006 he was elected Speaker of the Riksdag by the new liberal-conservative majority, defeating incumbent Björn von Sydow.

[6] On 4 October 2010 he was dramatically re-elected as Speaker of the Riksdag, by a 194–153 vote (with 20 votes in favor being from the Sweden Democrats, whose potential decisiveness to his re-election had been viewed as controversial).