[3] On 11 December 2014 he was elected as Chief Minister after his predecessor, Katy Gallagher, resigned and announced her intention to run for the Senate.

[4] In addition to being Chief Minister, he holds the portfolios of Treasurer; Climate Action; Trade, Investment and Economic Development; and Tourism.

After graduating in 1995, Barr worked for the Federal parliamentarian Annette Ellis and later the then ACT Opposition Leader Jon Stanhope, before embarking on a career change to the private sector in 1999 as a media analyst and account manager.

He campaigned largely on financial management, housing affordability and urban infrastructure issues, such as balanced territory budgets, tax concessions for first home buyers, inner city parking permits and footpath and street lighting upgrades, while also expressing support for gay law reform, the reproductive rights of women, multiculturalism and refugees.

[citation needed] Barr polled strongly in inner city booths and ultimately found himself in contention for the seventh and final Molonglo seat, along with Liberals Jacqui Burke, Zed Seselja and fellow Labor candidate Mike Hettinger, but narrowly missed out, coming in tenth.

Barr continued working for Hargreaves after the election, but received a second chance at entering the Assembly when the Treasurer, Ted Quinlan, resigned in March 2006.

Quinlan's resignation from the Assembly sparked a ministerial reshuffle, with Barr immediately entering the ministry upon taking his seat.

[15] This program includes the abolition of stamp duty on property transfers and insurance premiums, with the foregone revenue replaced through the general rates system—an approach supported by most economists.

[17][18] As Treasurer and Chief Minister, Barr has focused on the ACT's economic development, particularly encouraging private sector investment in higher education, trade, tourism, major events, arts and sport.

[26] In September 2019, the Barr Government approved the business case for stage 2A of the light rail, which is to extend the line with three additional stops from the city centre to Commonwealth Park.

Expected to be completed in November 2024, this new ticketing system will allow for more accurate real-time location transport data and the ability to pay with credit cards, smartphones, and wearable devices.

[29][30][31] Barr's government has continued its predecessors' climate change policies, including reducing carbon emissions and increasing the use of renewable energy.

[33][34] In 2020, the ACT became the first major jurisdiction outside of the European Union to achieve net-zero greenhouse gas emissions electricity supply.

In 2017, Barr was a prominent advocate for the 'Yes' campaign in the Australian Marriage Law Postal Survey, in which the ACT had the highest Yes result (74%) and participation rate (82.5%) in Australia.