In the 2004 ACT Party leadership election, he backed Stephen Franks to succeed Richard Prebble, over the eventual winner Rodney Hide.

[5] Before entering parliament he was best known for his campaign against the Electoral Finance Act, for which he organised protests and legal action and spent more than NZ$140,000.

[6] In 2008, Boscawen was ranked fourth on the ACT party's list and also stood, unsuccessfully, in the North Shore electorate.

Boscawen sat on the Finance and Expenditure, Commerce, and Parliamentary Service select committees, and was ACT's spokesperson for a range of issues including Housing, Transport, Energy and Economic Development.

[7] On 17 August 2010, Boscawen challenged Roy for the deputy leader role and was successful with a 3–2 caucus vote.

[4][8] This decision followed Roy's attempt the previous year, which had been supported by ACT MP and co-founder Sir Roger Douglas, to replace Hide as party leader.

[3] Boscawen declined to stand on the party list but contested the electorate vote in Tāmaki, which he, accurately, did not expect to win.