[5] In 2008, McClay was selected as the National Party candidate to stand for the Rotorua electorate in the 2008 New Zealand general election, running against incumbent Labour MP Steve Chadwick.

[10] In late June 2012, McClay announced his intention to bring a bill before Parliament to prohibit the display of gang insignia in all government premises, schools and hospitals in New Zealand.

[16] In September 2014, McClay was again elected as Member of Parliament for Rotorua with an increased majority of 7,418, after beating Labour candidate and former TV weatherman, Tāmati Coffey.

[19] In late July 2016, McClay was rebuked by Prime Minister John Key for downplaying concerns that China would retaliate if New Zealand undertook an investigation of Chinese steel dumping.

[28][29] Following the announcement of Todd Muller's retirement from Parliament on 17 March 2023, McClay was given National's Agriculture portfolio and promoted to 12th place ranking within Luxon's shadow cabinet.

[31] In late November 2023, McClay was sworn in as a Minister taking responsibility of the Agriculture, Forestry, Hunting and Fishing, and Trade portfolios.

[35] On 14 June, McClay announced that the Government would cancel the annual service charges for forestry under the country's emissions trading scheme, which amounts to NZ$30.25 per hectare.

[36] On 30 July 2024, McClay apologised after telling Green Party MP Ricardo Menéndez March "you're not in Mexico now, we don't do things like that here" during a parliamentary sitting.

Opposition leader Chris Hipkins described McClay's remarks as offensive towards migrant New Zealanders and questioned his suitability to remain as trade minister.

These free trade agreements lifted duties on the vast majority of New Zealand's exports to these Gulf Arab states over the next three to ten years.

[39][40] On 13 January 2025, McClay signed the comprehensive economic partnership agreement with the United Arab Emirates, cutting tariffs on 98.5% of New Zealand exports to that country.