Peter has been a Labor Party member of the Western Australian Legislative Assembly since November 2009, representing the electorate of Willagee.

In the same year, he was appointed a Member of the Order of Australia (AM) in the military division for "dynamic leadership and consistent professional excellence".

[1][2][3] In late 2006, after leaving the army with the rank of major, Tinley spoke out against the decision of the Howard government to support the US and British initiative to invade Iraq on the basis of purported evidence of the country's possession of weapons of mass destruction.

[6] The sitting member, Liberal MP Michael Keenan, retained the seat with a slightly reduced majority.

Five days later on 22 March, Tinley resigned as Mines and Petroleum minister, to avoid any claim of conflict of interest due to a family member working in the industry, and exchanged the Housing portfolio with Bill Johnston.