After graduation from university, he worked as a geologist for the Department of Mines and Energy of the Government of Queensland. Wallin discovered coal deposits throughout Queensland. In 1989 he founded QCoal for the purpose of developing these deposits into operating mines. QCoal mines coal from a number of mines in the Bowen Basin, including the Sonoma mine, Drake mine and Byerwen mine.
He lives in The Gap, a suburb of Brisbane. In 2013 he partnered with the Royal Flying Doctor Service to deliver a mobile dental service to provide free dental health care to communities across central and northern Queensland.
In January 2019Forbes estimated Wallin's net worth as 910 million., Wallin's net worth was assessed by the Financial Review on the 2025 Financial ReviewRich List as 2.06 billion.