McCallum's father, Ron McCallum, was the electorate chairman for the National Party in Kaipara and Whangārei.[1] McCallum moved to Northland in the 1970s. He studied agricultural commerce at Lincoln College,[2] graduating in 1988.[3] He owns a farm in Maungaturoto.[1] He is the chairperson of the Kauri Museum in Matakohe.[1] McCallum has been on the executive of the Bluegreens (the National Party environmental issues advisory group) since 2001, spent ten years serving on the National Party board (2005–2015), and was vice-president of the Northland branch of Federated Farmers.[1][4]
McCallum contested the selection for the National Party candidacy for the 2015 Northland by-election, but lost out to Mark Osborne.[5] McCallum was the campaign chairman for MP Matt King in the 2020 general election, when King lost his seat.[6] After joining the 2022 protests at Parliament, King resigned from the National Party.[7]
McCallum identified the cost of living and regulatory burden on farmers as issues in his electorate.[1] After the election, he said "It’s a huge responsibility and Northland has quite a lot of challenges and big issues that we can’t fix overnight. We need to see what state the economy is in and we are going to have to really justify any spending we do."[10]