Sean Bell, a former manual labourer, was picked by One Nation to fill a casual vacancy in the Senate left by former hostage negotiator Warwick Stacey, who resigned months after winning the NSW seat in the May election.
It was the right-wing minor party's best election result in terms of seats won, outside of a double dissolution.
Senator Hanson, who founded and continues to lead the party in federal parliament, said Mr Bell would bring a wealth of parliamentary and political experience to the role after serving as her adviser for nine years.
"He was instrumental in our successful campaign to defeat the voice to parliament, as well as in delivering our strongest-ever result at the federal election earlier this year," she said on Tuesday.
"I have every confidence he will represent the people of NSW with conviction, skill, dedication and the conservative values which made this nation great."
In a statement released by One Nation, the 38-year-old was described as a husband and father with a background in retail, hospitality and manual labouring.
Mr Bell pursued further education and started a business as a claims assessor before entering politics.
"I'm fortunate to have worked closely with one of Australia's most prominent and experienced political leaders in Senator Hanson, and I'm looking forward to bringing this experience to the Senate in the interests of the people of NSW," he said.
He listed his priorities as ending mass immigration, wasteful spending and "ideological agendas that punish working Australians".
High levels of immigration were driving demand beyond what NSW could handle, causing housing to become unaffordable, overloading infrastructure and driving up energy prices, Mr Bell claimed.
He also took aim at net-zero policies, which he said were worsening cost-of-living concerns.
"Too many people in regional NSW feel completely ignored by politicians in Canberra," Mr Bell said.
"I want to see a government that puts Australian families, industries, and values first and restores pride in our nation."
Rancour over immigration and cost-of-living pressures has helped One Nation build on its popularity as the Liberal Party brand continues to flounder.
Nationwide rallies drew thousands of anti-immigration protesters at the end of August and were supported by One Nation senators.
A survey published by pollster Resolve in Nine newspapers on Sunday revealed One Nation's national vote surged to 12 per cent - the highest share the party has received in the poll.
In NSW, One Nation soared to 16 per cent, leapfrogging the Greens to become the third largest party in Mr Bell's home state.
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