Neilson started his financial management career in the Investments department of Courtaulds in London. After returning to South Africa in 1973, he moved to Australia ten years later as the head of Retail funds management for Bankers Trust Australia (now BT Australia). Neilson established the Platinum Asset Management fund in 1994 with initial backing from George Soros. Platinum specialises in international equities and has an estimated $US16 billion in funds under management. Neilson is its managing Director and portfolio manager. In May 2007, Neilson floated 20 percent of Platinum Asset Management on the Australian Stock Exchange. The 57 percent of the company's shares he retained were valued at A$2.9 billion, making him one of Australia's richest people.
Neilson and his former wife, Judith, have two children. In 2009, Kerr and Judith Neilson opened the White Rabbit Gallery in Sydney to share with the public their extensive collection of contemporary Chinese art. Kerr and Judith Neilson divorced in 2015. Kerr has been listed on the BRW Rich 200 since 2011. Following his divorce, according to the BRW his net worth fell from A$3.35 billion to A$2 billion in 2015, and to A$1.95 billion in 2016. Meanwhile, his former wife, Judith Neilson debuted on the 2015 BRW Rich 200 with a net worth of A$1.55 billion in 2015, and was assessed as having a net worth of A$1.45 billion in 2016.
Kerr Neilson first appeared on the Forbes list of Australia's 50 richest people in 2010 with a net worth of US$2.20 billion. In the latest list that was published in early 2016, following his divorce, Forbes assessed Neilson's net worth at US$1.85 billion.
Neilson is regularly likened to Warren Buffett for his ability to consistently select high-performing stocks. However, poor results during the 2012 financial year resulted in a 16 per cent fall in net profit, mainly due to a 14 per cent reduction in investment income. As a result, Neilsen agreed to forego a performance bonus, an increase in his base salary, and neither granted himself nor exercised options. As the principal shareholder of Platinum, Neilsen yielded A$42 million in dividends during the 2012 financial year.