Politics and parliament
L'Estrange joined the Liberal Party of Western Australia in 2002 and held various pre-parliamentary service positions in the party.
In 2013 L'Estrange was elected to the seat of Churchlands in the 39th Parliament of Western Australia. The seat had been held by conservative independent Liz Constable since 1991, dating to its time as Floreat. However, it was located in the Liberals' heartland of the southwest beachside suburbs of Perth, and it was a foregone conclusion that it would revert to the Liberals once Constable retired. As expected, L'Estrange won comfortably, with 59 percent of the primary vote and 70.2 percent of the two-party vote.
L'Estrange was a member of the Joint Parliamentary Standing Committee on the Corruption and Crime Commission, Chairman of the Public Accounts Committee, Deputy Chairman of the Joint Standing Committee on Audit, and a member of the Parliamentary Liberal Party's Finance, Commerce & Development Committee. In March 2016, he was appointed Minister for Mines and Petroleum and Minister for Small Business. In September 2016 he was also appointed as Minister for Finance and to the Government's Economic and Expenditure Reform Committee.
In 2017, L'Estrange was re-elected to the seat of Churchlands in the 40th Parliament of Western Australia. The Liberal Party suffered a primary vote swing of 16 percent but L’Estrange kept his swing to 4 percent, securing a comfortable TPP majority of 63.2 percent, the second-safest Liberal seat in Perth. Following the Liberal Government's defeat, he was appointed Manager of Opposition Business in the Legislative Assembly and Shadow Minister for Mines and Petroleum; Economic Development; Defence Issues; Innovation and ICT; Science. He was also elected as Deputy Chairman of the Parliament's Economic and Industry Standing Committee. In January 2018 he was made Shadow Minister for Health and Mental Health. In June 2019 he was appointed Shadow Minister for Corrective Services and Housing and elected to the Parliament's Community and Justice Standing Committee. In February 2020 he took on the role as Shadow Minister for Corrective Services; Emergency Services; Defence Issues; and Racing and Gaming. In December 2020 he was made Shadow Treasurer.
At the 2021 election L'Estrange was caught up in the Liberal Party landslide defeat set amongst the backdrop of COVID-19; however, it was to be a significant boundary change - removing the blue-Liberal area of central City Beach and replacing it with the Labor-red area of Scarborough-Doubleview that saw 205 voters preference flows defeat him. The seat of Churchlands remained the second best performing Liberal seat in metropolitan Perth with a winning primary vote of 44%; however, the final TPP result was 49.19% to Labor 50.81%. Only the seat of Cottesloe was retained by the Liberal Party in metropolitan Perth.