Sir David Barnes, who has died aged 84, was the self-effacing but determined and clear-sighted chief executive who turned the bioscience interests of ICI into one of the world’s major pharmaceutical corporations, AstraZeneca.

Teased at its launch in 1993 that Zeneca sounded like a Czechoslovakian camera, Barnes responded that its performance would define its brand – and was vindicated. The first suggested name had been Zenica, but then Barnes, tracking the Bosnian conflict days before the launch, found to his horror that hostilities were threatening to spread to a previously unremarked town of that name. Alarmed that it “could become as notorious as Guernica”, he changed the spelling and held his breath.

It was one of a number of courageous decisions – another being that Zeneca should not rush into a merger until the right partner (which turned out to be the Swedish Astra company) came along – that marked out Barnes’s career.

The son of Eric Barnes, a colonial official, and his wife, Jean, David was born in Blantyre, which was then in Nyasaland and is now Malawi’s second largest city. At six he was sent on a perilous journey to prep school in wartime England. He was educated at Shrewsbury school, and during national service saw action in the Malayan emergency. With a passing resemblance to the young Dirk Bogarde, he sometimes seemed the quintessence of a 1950s Englishman pulling on his pipe and seeing no contradiction between enjoying both birdwatching and duck-shooting.

At Shrewsbury, where he commanded the cadet force, a fascination with animal behaviour led him to long sessions dissecting in the biology labs before he enrolled for a veterinary degree at Liverpool University. The surprise was that he never finished, having been advised by his dean that he lacked application; Barnes’s explanation was that he was too involved in university debating and reluctant to enter a profession so often required to kill its patients.

Instead, he filled in time before national service as a lab assistant at the newly formed ICI pharmaceutical division at Alderley Edge, Cheshire. In 1960 he rejoined in the more substantive post of a commercial assistant and within four years was a section head in European sales. He met his future wife, Fiona Riddell, at Alderley and they married in 1963.

In 1971 he became overseas director, working closely with the chairman Peter Cunliffe to build the division into a genuinely international business, becoming deputy chairman in 1977. His qualities were recognised by the wider ICI group with appointment in 1983 as chairman of its paints division. Barnes was credited once again with transforming a business into a genuinely international player, crowned by the acquisition of the US-based Glidden Paints in 1986, the year he was promoted to the ICI board.

ICI had successfully grown its own pharmaceutical division, with world-beating heart and cancer drugs, but it competed with the older, capital-intensive heavy chemical businesses for investment. As the only commercial director who had grown up in the business, Barnes was a key advocate for its expansion. When ICI reshaped under John Harvey-Jones to favour the newer businesses, Barnes continued to argue for more decisive action, “hanging on to the jewels and getting rid of the tarnished tinsel”.

A passionate advocate of applied science, Barnes chaired the pharmaceutical economic development council of the National Economic Development Office, charting the issues for the industry for a decade until 1992 . He was an outspoken champion of research into genetic modification, which he saw as vital to feed the developing world. He was proud that ICI’s development of GM tomatoes led to the first GM foodstuff – bestselling tomato paste – on UK supermarket shelves, calling the decision by supermarkets subsequently to withdraw it as “a bit pusillanimous”.

The cyclical boom in heavy chemicals in the mid-1980s helped ICI to become the first UK company to post a £1bn profit but obscured their longterm weakness. However, when Lord Hanson took his run at ICI after the 1990 recession, it was obvious that not enough reshaping had been done and ICI management was spread over too many businesses. When the board took the bold decision in 1993 to demerge the bioscience business, primarily pharmaceuticals and agrochemicals, Barnes was in the right place at the right time. As the only senior director with a strong pharmaceutical background he was the obvious choice for chief executive.

He assumed command with the clarity of the good military commander he had once aspired to be. The new company would, he said, be “bold and innovative” and while he genuflected to “the best of the ICI heritage” he made it clear that things would be done differently, establishing a relatively small head office, with fewer committees. His personal approachability and lack of pomposity set a refreshing tone. Pharmaceutical executives who had chafed under ICI control felt they were given freedom. One, Sir Tom McKillop, later chairman, described it as “having a great stone lifted from our backs”.

Zeneca’s first months were marked by criticism, rebutted by Barnes, of its drugs pipeline – the potential of the drugs passing through its development labs – and its shares initially trailed ICI’s. But under his calm direction Zeneca began to reach his target – recognition as one of the major quoted pharmaceutical companies in the world. Having been made CBE in 1987, he was knighted in 1996.

Initially he resisted regular overtures for mergers, sometimes characterising rival alliances as “two drunks holding each other up”, and welded a disparate group of executives into a harmonious team.

He was reluctant to move away from the “three-legged stool” model of a bioscience business incorporating agrochemicals and specialty chemicals as well. But, once persuaded, he could move decisively. The specialties business was divested in 1998, and by 1997 the Swedish drugs firm Astra had been identified as a preferred partner. Talks were deferred while Astra attended to issues with its US subsidiary, but Barnes stayed in touch and when negotiations resumed it was the good relations he had established with his Astra counterparts that were decisive in affecting the merger. In particular, his decision as Zeneca chairman-elect to step back and take a supporting role, as one of two deputy chairmen, defused touchy corporate governance issues.

Barnes stayed on for only two years, though he remained deputy chairman of Syngenta, the spinoff of AstraZeneca’s and Novartis’s agrochemical interests, until 2004.

He directed his energies towards supporting his family and a range of pro bono activities. He was a governor of Ashridge business school in Hertfordshire, a deputy chairman of Business in the Community, and served on the governing bodies of his old school and university. He was active in the British Veterinary Association and cancer research and raised funds for the Thames Valley hospice.

Barnes performed one more service for AstraZeneca when it was threatened by takeover by Pfizer in 2014. Emerging from retirement with a typically trenchant phrase he declared his concern that Pfizer would “act like praying mantis and suck the lifeblood out of their prey”. He took the view that if Pfizer wanted the UK tax breaks they should build their own labs in Britain and not do it via a takeover. Pfizer walked away after its final offer was refused.

Always a countryman, who declared he could never live in a town, Barnes took pleasure in restoring a narrowboat as well as fishing, shooting and walking.

He is survived by Fiona and their son, Jonathan, and daughter, Alison.

• James David Francis Barnes, businessman, born 4 March 1936; died 4 March 2020