In 1987, Hart completed an MBA from the University of Otago. His research thesis outlines the strategy for Rank, then a small hire company, to grow. This strategy relies on using the cash flow of well-performing companies to fund debt, which as it gets paid off, increases the equity value of the initial Investors.
Hart gained a big break when he purchased the Government Printing Office for less than its capital value in 1990. The purchase was 1.4x earnings and Hart was provided generous payment terms. Then New Zealand Prime Minister David Lange initially refused to sign off on the transaction. The following year he bought Whitcoulls Group which at that time included a Retail chain of bookstores as well as office and stationery concerns. He has since sold off these interests.
Burns Philp and Company Limited was an Australian and New Zealand food Manufacturing company dual listed on the ASX and NZX. Hart has been the chairman since September 2004 and a member of the board of Directors since September 1997. In 2003 Burns Philp performed a A$2.4bn hostile takeover of the much larger food group Goodman Fielder before relisting it through an IPO.
In 2006 Hart paid NZ$3.3bn for Carter Holt Harvey (CHH), a New Zealand timber and paper Business. Soon after completing the purchase he began restructuring the struggling company starting with the sale of CHH's forests to US-based Hancock Timber Group for up to NZ$2bn. Hart has also sold CHH's head office property, various sawmills and packaging plants for over NZ$300M.
In 2007 he announced the sale of CHH's building supplies Business which some estimate could fetch NZ$2.3bn, but was unsuccessful in the selling of it.
In 2008 Hart paid US$2.7bn for Alcoa (AA) Packaging & Consumer group. He spun off the company and renamed it Reynolds Packaging Group, which is now headquartered in Lake Forest, Illinois, USA. While this Business is within the packaging industry, it is not paper packaging related, being mostly in the aluminium foil and plastic closure Business. Since the purchase from Alcoa, Hart has cut more than 20% of the workforce within Reynolds, mostly through plant shutdowns, including the flagship Reynolds Wrap Foil Plant in Richmond, Virginia and restructuring efforts. This has resulted in significant savings and profit margin jump for the company, allowing him to issue more debt on behalf of the company in October 2009 and get a high return on his initial investment.
In November 2009 and May 2010, Hart, through additional debt financing has combined the packaging groups he owns into Reynolds Group Holdings Limited. RGHL is the combination of four operating segments: SIG (a beverage packaging manufacturer headquartered in Zurich), Closure Systems International (a plastic bottle cap manufacturer headquartered in Indianapolis), Evergreen Packaging (a beverage packaging manufacturer headquartered in Memphis), and Reynolds Consumer Products(an aluminium foil and other packaging materials manufacturer) located in Lake Forest, Illinois. While the operations are spread around the world, the RGHL corporate headquarters are located in the same office building as Reynolds Consumer Products in Lake Forest, Illinois.
Hart had also been seeking a purchaser for the packaging side of CHH since October 2010. In April 2014, CHH announced the sale of its Pulp, Paper & Packaging Business to a Japanese consortium for NZ$1.037bn, with the deal closing in the second half of 2014.
On June 17, 2011, Reynolds Group Holdings announced its intention to acquire all of the outstanding stock of Graham Packaging Company, Inc, headquartered in York, PA. The acquisition was completed on September 8, 2011 for $25.50 per share (in cash), for a total enterprise value, including net debt, of approximately USD$4.5B. This acquisition increased Rank's growing profile in the US. Earlier in 2011, it agreed to buy the Automotive consumer Business of Honeywell International (the FRAM group) for $950M USD. In March 2015 Reynolds Group Holdings completed the sale of SIG to Onex Corporation. 
While he prefers to keep a low profile in the general media he was notable for the launch of his 58 m luxury motor yacht Ulysses at Auckland's Viaduct Harbour in January 2006. The yacht is valued at nearly $100M and took five years to complete due to being gutted by a fire during refit by a New Orleans shipyard. In 2012 it was put up for sale for $49-million. Hart has a new yacht under building at Kleven Verftin the community of Ulstein in Norway. The yard specialises in off-shore vessels and the 107-meter long yacht is constructed to sail under rough conditions, equipped with helicopter deck, hangar and accommodation for 60 persons. Hart's latest 107-metre "expedition yacht," formerly named Ulysses, was completed and eventually launched on 3 September 2014, in Ulsteinvik, north of Oslo, at the Kleven Verft shipyard.
In March 2015 Reynolds Group Holdings completed the sale of SIG to Onex Corporation.
Forbes stated that Hart was the 178th richest person in the world as of March 2016.
In 2017 it was reported that he purchased a new 116m Explorer yacht, which he has named "Ulysses", now that his 107m yacht, formerly of the same name, has been sold.