Ashcroft collects Victoria Crosses which are awarded for valour and devotion to duty to members of various armed forces. His collection is by far the largest in the world spanning 128 years from acts of valour at the start of the Crimean War in 1854 to an act of courage during the Falklands War in 1982. He wrote Victoria Cross Heroes, to mark the 150th anniversary of the Victoria Cross.

After a period in Belize after completing his education, Ashcroft returned to hitch hike around Europe, and then for a short period of time became the manager of a rock and roll band. In 1967, Ashcroft joined Carreras Tobacco as a management trainee.

He left in 1969, joining Pritchard Group Services, after several months unemployed. Pritchard Group was a cleaning and Business services company and Ashcroft worked his way up quickly to become an assistant in the company's head office accounting department, part of a large acquisitions team.

In April 1972, Ashcroft married Wendy Burrell in Hampstead, north London. They had two sons and one daughter. His second marriage in 1986 was to Susan Anstey. The couple have homes in London, Maidenhead in Berkshire, and Belize.

On exiting Uni-Kleen in 1977, his next purchase was Hawley Goodall, another poorly performing company, this time in camping equipment manufacture. Ashcroft used Hawley to make a series of acquisitions, transforming Hawley into a Business services group, ranging from janitorial services for hospitals and offices, to car auction services, and later with a focus on the security services industry. Through the sale of the car auctions division to the fast-expanding British Car Auctions, he formed a lifelong friendship with David Wickins, whom he would later help take a majority stake in Lotus Cars, as well as Finance other joint-ventures. By 1981, Hawley had made its first acquisitions in the United States, and its total revenues had grown to $27 million.

By the late 1980s, BHI had become one of the largest holding companies in Belize, with direct interests in or holdings via main operating company Stargate Ltd, ranging from telecommunications, property, the Belize shipping register, and citrus fruits.

Barrow also warned David Cameron that relations with the United Kingdom would be damaged if Ashcroft were given a senior position in government. In 1981, Belize had gained independence from the UK. Seeing the opportunity to build an off-shore operations base and control the country's financial Service, in 1984 Ashcroft formed Belize Holdings (BHI), which became the vehicle for a parallel acquisition spree during the 1980s, beyond the scope of Hawley.

By early 1983, Hawley had built up a 20% stake in pharmaceutical packaging manufacturer Cope Allman. Ashcroft offered to increase his stake to 29.9%, just below the 30% level at which a formal bid for the entire company must be launched. Ashcroft and Cope Allman fought bitterly over the purchase share price and current holdings, with Cope Allman reporting Ashcroft and Wickins to the Takeover panel, after discovering that BCA had built up a 13.5% in the company. But the takeover panel found that the Ashcroft and Wickins were operating independently, so Hawley was able to increase its holding to 29.9%. At this point the combined holdings of Hawley/BCA in Cope Allman amounted to 43.5% per cent of Cope Allman, giving them the power to introduce sweeping changes without launching a full bid. Cope Allman was eventually sold to an MBO backed by Hawley and financed by Bain Capital, and then sold to Bowater in 1992 in a complex swap of assets with ADT/Hawley.

In 1985 Ashcroft and Wickins bought car sales dealership Henlys Group via a Canadian-registered company, Mipec. Controlled by Ashcroft's Hawley Goodall, Henlys was merged with the already owned funeral hearse maker Coleman Milne to form a Motoring Division. In 1989, Hawley Goodall sold its Motoring Division consisting of Henlys and Coleman Milne to the Plaxton Group, the bus and coach manufacturer based in Scarborough, North Yorkshire.

In 1986, Hawley bought out Ashcroft's former employer, Pritchard Services, leaping to the second place in the U.S. services industry. At this time, Hawley had revenues of more than $1.3 billion.

In 1987, BHI led the formation of Belize Bank Holdings (BBH), which took control of Belize Bank from the Royal Bank of Canada. Belize Bank has become the country's largest financial institution, controlling some 50 percent of the market. BBH developed local and international interests in facilities services, Finance and telecommunications. Belize Bank itself formerly held a majority stake in Belize Telemedia Limited (BTL), until it was nationalised by the Government of Belize.

In 1997, ADT was sold by a reverse takeover to US conglomerate Tyco International for $6.7 billion, allowing Tyco to become tax-efficient.

Ashcroft has close Business and other connections with the Commonwealth country of Belize, and served as the country's ambassador to the United Nations between 1998 and 2000. In his 2005 biography, he admitted that it is a country where his interests have been "exempt from certain taxes for 30 years." In 2009, the Prime Minister of Belize Dean Barrow told its parliament:

In 1999, he was given the Honorary Degree of Doctor of the University by Anglia Ruskin University, he would be named Chancellor of the University in 2001.

In the 2000 Queen's Birthday Honours, on the advice of the Belizean government, he was appointed Knight Commander of the Order of St Michael and St George (KCMG) "for public Service to the community and country" of Belize.

Ashcroft has been Chancellor of Anglia Ruskin University since November 2001. He donated £5 million in 1999 for the university's Business school at Chelmsford, now called Lord Ashcroft International Business School, and another £5million gift in 2009 to create a new Business school in Cambridge.

Ashcroft disposed of large amounts of the Tyco stock which he had acquired as a result of the sale of ADT, explaining that he needed the capital to diversify into other things and that he never retained a substantial stake in any enterprise which he did not control. Ashcroft nevertheless continued as a non-executive Director of Tyco, a role he still held in 2002 when Tyco CEO Dennis Kozlowski was arrested in New York in connection with personal tax offences. Unease had already been expressed at Tyco at some of Kozlowski's corporate decisions and Ashcroft was amongst the Directors who appointed Lawyer David Boies to investigate irregularities in the company. In time, the exposure of management deficiencies led to Ashcroft's demanding that the whole of the board of Directors of Tyco should resign, to be replaced by new management.

In 2003, Ashcroft was criticised by the High Court judge, Mr Justice Peter Smith in Rock (Nominees) Ltd v RCO (Holdings) Plc. Smith condemned Ashcroft's tactics in relation to the takeover of cleaning company RCO by the Danish firm ISS. Smith said,

In 2004, he clashed with Conservative leader Michael Howard when he offered a £2m donation on the condition that it should go to his specified candidates rather than into general Conservative Central Office funds. Ashcroft stated in 2005: "I much prefer to be involved, to make sure that my investment is wisely placed."

In December 2005, he was appointed Deputy Chairman of the Conservative Party.

Ashcroft is a whale spotter, through which interest he has financially supported various environmental groups and campaigns. He financially supported the Environmental Investigation Agency, who persuaded him to back a television campaign in six Caribbean countries, aimed at coaxing them to withdraw their support for whaling. The countries had received $16 million (£8 million) a year in fisheries aid from Japan. The campaign coincided with the 2006 International Whaling Commission conference in St Kitts.

On 12 October 2007 he was accused by Labour MPs of being allowed to heavily fund local Conservative organisations in marginal seats of his choosing. Lord Ashcroft has insisted such funding is legitimate.

In 2008, The Economist ran an article on the web of loans and court cases surrounding Belize Bank (owned by Ashcroft), a private hospital company called Universal Health Services and the government of Belize. It was alleged that Belize Bank had wrongly appropriated monies sent from the government of Venezuela for housing purposes in order to settle debts relating to loans to UHS and illegally guaranteed in secret by the Belizean premier.

On 12 October 2009, Lord Ashcroft pledged NZ$50,000 for the safe return of two-year-old toddler Aisling Symes. Aisling had gone missing a week earlier in West Auckland.

He was nominated by Conservative party leader william Hague on the condition that he became a UK resident although at the beginning of 2010 he announced his "non-domiciled" tax status. Ashcroft had announced that he intended to take the title "Baron Ashcroft of Belize", a suggestion that infuriated his political opponents. He later said this had been a joke, and his title was created as simply "Baron Ashcroft".

Ashcroft has also supported military causes; in September 2011 he hosted a reception at the House of Lords for the launch of the Felix Fund, the charity for bomb disposal experts and their families.

He was sworn in as a member of the Privy Council of the United Kingdom in 2012, entitling him to use the post-nominal letters "PC" for life.

In 2013, Ashcroft stated that he has given tens of millions of pounds to good causes over the years. In May 2013, he pledged to donate more than half of his money to good causes, and became a signatory of The Giving Pledge.

On 31 March 2015, the day after the prorogation of Parliament ahead of the 2015 general election, he announced his resignation from the House of Lords with immediate effect, stating he would continue in politics.

Ashcroft has been described as a "tax exile", and for a number of years lived in the UK enjoying the controversial non-domiciled tax status. At the time of his ennoblement the Cabinet Office said that Ashcroft would be taking up permanent residence in the UK for tax purposes, an undertaking described in the newspapers as a "pledge" and a "gentleman's agreement", but he did not in fact claim to do so until a decade later, when a change in the law would have required him to quit the Lords, had he not done so. Then in 2017 it was revealed following the Paradise Papers leak of offshore investment documents that Lord Ashcroft remained a domicile of Belize despite having claimed to have given up his non-dom tax status in 2010.