Fiennes was born in Windsor, Berkshire on 7 March 1944, nearly four months after the death of his father, Lieutenant-Colonel Sir Ranulph Twisleton-Wykeham-Fiennes. Whilst commanding the Royal Scots Greys in Italy Fiennes’ father trod on a German anti-personnel S-mine and died of his wounds eleven days later in Naples on 24 November 1943. He was posthumously awarded the Distinguished Service Order. Fiennes’ mother was Audrey Joan (died 2004), younger daughter of Sir Percy Newson, Bt. Fiennes inherited his father’s baronetcy, becoming the 3rd Baronet of Banbury, at his birth.

After graduating from the Mons Officer Cadet School on 27 July 1963 Fiennes served in his father’s regiment, the Royal Scots Greys, and was seconded to the Special Air Service where he specialised in demolitions.

Since the 1960s Fiennes has been an expedition leader. He led expeditions up the White Nile on a hovercraft in 1969 and on Norway’s Jostedalsbreen Glacier in 1970. One notable trek was the Transglobe Expedition he undertook between 1979 and 1982 when he and two fellow members of 21 SAS, Oliver Shepard and Charles R. Burton, journeyed around the world on its polar axis, using surface transport only. Nobody else has ever done so by any route before or since.

Fiennes married his childhood sweetheart Virginia (“Ginny”) Pepper on 9 September 1970. They ran a country farm estate in Exmoor, Somerset, where they raised cattle and sheep. Ginny built up a herd of Aberdeen Angus cattle while Fiennes was away on his expeditions. The extent of her support for him was so great that she became the first woman to receive the Polar Medal. The two remained married until her death from stomach cancer in February 2004.

In 1970, while serving with the Omani Army, Fiennes received the Sultan’s Bravery Medal. He has also been awarded a number of honorary doctorates, the first in 1986 by Loughborough University, followed in 1995 by University of Central England, in 2000 by University of Portsmouth, 2002 by Glasgow Caledonian University, 2005 by University of Sheffield, 2007 by University of Abertay Dundee and September 2011 by University of Plymouth. Fiennes later received the Royal Geographical Society’s Founder’s Medal.

Becoming disillusioned by his British Army service, in particular his career prospects, he spent the last two years of his service seconded to the army of the Sultan of Oman. At the time, Oman was experiencing a growing communist insurgency supported from neighbouring South Yemen. Fiennes had a crisis of conscience soon after arriving in Oman, as he became aware of the Sultan’s poor government. However he decided that the oppression threatened by a communist takeover, combined with moves towards progressive change within the Sultanate system, justified his part in the conflict. After familiarisation, he commanded the Reconnaissance Platoon of the Muscat Regiment, seeing extensive active service in the Dhofar Rebellion. He led several raids deep into rebel-held territory on the Djebel Dhofar and was decorated for bravery by the Sultanate. After eight years’ service Fiennes relinquished his commission on 27 July 1971.

As part of the Transglobe Expedition, Fiennes and Burton completed the Northwest Passage. They left Tuktoyaktuk on 26 July 1981, in an 18 ft open Boston Whaler and reached Tanquary Fiord on 31 August 1981. Their journey was the first open boat transit from West to East and covered around 3,000 miles (2,600 nautical miles or 4,800 km) taking a route through Dolphin and Union Strait following the south coast of Victoria Island and King William Island, north to Resolute Bay via the Franklin Strait and Peel Sound, around the south and east coasts of Devon Island, through Hell Gate and across Norwegian Bay to Eureka, Greely Bay and the head of Tanquary Fiord. Once they reached Tanquary Fiord, they had to trek a further 150 miles via Lake Hazen to Alert before setting up their winter base camp.

He was the subject of This Is Your Life in 1982 when he was surprised by Eamonn Andrews.

In 1986 Fiennes was awarded the Polar Medal for “outstanding service to British Polar exploration and research.” In 1994 he was awarded a second clasp to the Polar Medal, having visited both poles. He remains the only person to have received a double clasp for both the Arctic and Antarctica.

In 1992 Fiennes led an expedition that discovered what may be an outpost of the lost city of Iram in Oman. The following year he joined nutrition specialist Dr Mike Stroud to become the first to cross the Antarctic continent unsupported; they took 93 days. A further attempt in 1996 to walk to the South Pole solo, in aid of the Breast Cancer Campaign, was unsuccessful due to a kidney stone attack and he had to be rescued from the operation by his crew.

Fiennes was appointed Officer of the Order of the British Empire in 1993 for “human endeavour and for charitable services”: his expeditions have raised £14 million for good causes.

In 2000 he attempted to walk solo and unsupported to the North Pole. The expedition failed when his sleds fell through weak ice and Fiennes was forced to pull them out by hand. He sustained severe frostbite to the tips of all the fingers on his left hand, forcing him to abandon the attempt. On returning home, his surgeon insisted the necrotic fingertips be retained for several months before amputation, to allow regrowth of the remaining healthy tissue. Impatient at the pain the dying fingertips caused, Fiennes cut them off himself with an electric fretsaw, just above where the blood and the soreness was.

Despite suffering from a heart attack and undergoing a double heart bypass operation just four months before, Fiennes joined Stroud again in 2003 to complete seven marathons in seven days on seven continents in the Land Rover 7x7x7 Challenge for the British Heart Foundation. “In retrospect I wouldn’t have done it. I wouldn’t do it again. It was Mike Stroud’s idea”. Their series of marathons was as follows:

Fiennes’ career as an author has developed alongside his career as an explorer: he is the author of 24 fiction and non-fiction books, including The Feather Men. In 2003, he published a biography of Captain Robert Falcon Scott which attempted to provide a robust defence of Scott’s achievements and reputation, which had been strongly questioned by biographers such as Roland Huntford. Although others have made comparisons between Fiennes and Scott, Fiennes says he identifies more with Lawrence Oates, another member of Scott’s doomed Antarctic team.

In 2003, while attempting to scale Mount Everest Fiennes suffered a heart attack and later underwent emergency bypass surgery.

Fiennes stood for the Countryside Party in the 2004 European elections in the South West England region – fourth on their list of six. The party received 30,824 votes – insufficient for any of their candidates to be elected. Contrary to some reports, he has never been an official patron of the UK Independence Party. He is also a member of the libertarian pressure group The Freedom Association. In August 2014, Fiennes was one of 200 public figures who were signatories to a letter to The Guardian opposing Scottish independence in the run-up to September’s referendum on that issue.

In June 2005, Fiennes had to abandon an attempt to be the oldest Briton to climb Mount Everest when, in another climb for charity, he was forced to turn back as a result of heart problems, after reaching the final stopping point of the ascent. In March 2007, despite a lifelong fear of heights, Fiennes climbed the Eiger by its North Face, with sponsorship totalling £1.8 million to be paid to the Marie Curie Cancer Care Delivering Choice Programme. Kenton Cool first met Fiennes in 2004, and subsequently guided him in the Alps and Himalayas.

Fiennes embarked on a lecture tour, where in Cheshire he met Louise Millington, whom he married at St Boniface’s Church, Bunbury, one year and three weeks after Ginny’s death. A daughter, Elizabeth, was born in April 2006. He also has a stepson named Alexander. In 2007 Millington was interviewed by The Daily Telegraph to help raise money for the Philip Leverhulme Equine Hospital in Cheshire.

In the 2007 Top Gear: Polar Special the presenters travelled to the Magnetic North Pole in a Toyota Hilux. Fiennes was called in to speak with the presenters after their constant joking and horseplay during their cold weather training. As a former guest on the show who was familiar with their penchant for tomfoolery, Fiennes bluntly informed them of the grave dangers of polar expeditions, showing pictures of his own frostbite injuries and presenting what remained of his left hand. Sir Ranulph was given recognition by having his name placed before every surname in the closing credits: “Sir Ranulph Clarkson, Sir Ranulph Hammond, Sir Ranulph May”….

In May 2007 Fiennes received ITV’s Greatest Britons Award for Sport beating fellow nominees Lewis Hamilton and Joe Calzaghe. In October 2007 Fiennes ranked 94th (tied with five others) in a list of the “Top 100 living geniuses” published by The Daily Telegraph.

In 2008 Fiennes made his second attempt to climb Mount Everest, getting to within 400 metres (1,300 ft) of the summit before bad timing and bad weather stopped the expedition. On 20 May 2009 Fiennes reached the summit of Mount Everest, becoming the oldest British person to achieve this. Fiennes also became the first person ever to have climbed Everest and crossed both polar ice-caps. Of the other handful of adventurers who had visited both poles, only four had successfully crossed both polar icecaps: Norwegian Børge Ousland, Belgian Alain Hubert and Fiennes. In successfully reaching the summit of Everest in 2009 Fiennes became the first person ever to achieve all three goals. Ousland wrote to congratulate him. Fiennes continues to compete in UK-based endurance events and has seen recent success in the veteran categories of some Mountain Marathon races. His training nowadays consists of regular two-hour runs around Exmoor.

In late 2008/early 2009 Fiennes took part in a new BBC programme called Top Dogs: Adventures in War, Sea and Ice, in which he teamed with fellow Britons John Simpson, the BBC News world affairs editor, and Sir Robin Knox-Johnston, the round-the-world yachtsman. The team undertook three trips, with each team member experiencing the other’s adventure field. The first episode, aired on 27 March 2009, saw Fiennes, Simpson and Knox-Johnston go on a news-gathering trip to Afghanistan. The team reported from the Khyber Pass and the Tora Bora mountain complex. In the other two episodes they undertook a voyage around Cape Horn and an expedition hauling sledges across the deep-frozen Frobisher Bay in the far north of Canada.

On 6 March 2010, Fiennes was involved in a three-car collision in Stockport which resulted in minor injuries to himself and serious injuries to the driver of another car. He had been in Stockport to participate in the annual High Peak Marathon in Derbyshire as part of a veterans’ team known as Poles Apart that, despite the freezing conditions, managed to win the veterans’ trophy in just over 12 hours.

In 2010 Fiennes was named as the UK’s top celebrity fundraiser by Justgiving, after raising more than £2.5 million for Marie Curie Cancer Care over the previous two years – more than any other celebrity fundraiser featured on JustGiving.com during the same period.

In September 2011 Fiennes was awarded an honorary Doctorate in Science from Plymouth University and in July 2012 he was awarded an Honorary Fellowship from the University of Glamorgan.

In September 2012 it was announced that Fiennes was to lead the first attempt to cross Antarctica during the southern winter, in aid of the charity Seeing is Believing, an initiative to prevent avoidable blindness. The six-man team was dropped off by ship at Crown Bay in Queen Maud Land in January 2013, and waited until the Southern Hemisphere’s autumnal equinox on 21 March 2013 before embarking across the ice shelf. The team would ascend 10,000 feet (3,000 m) onto the inland plateau, and head to the South Pole. The intention was for Fiennes and his skiing partner, Dr Mike Stroud, to lead on foot and be followed by two bulldozers dragging industrial sledges.

In December 2012 Fiennes was named one of the Men of the Year for 2012 by Top Gear magazine.

Fiennes had to pull out of The Coldest Journey expedition on 25 February 2013 because of frostbite and was evacuated from Antarctica.

Most recently Fiennes was an expert guest commentator on the PBS documentary Chasing Shackleton which aired in January 2014. Fiennes makes a number of corporate and after dinner speeches.

In October 2014 it was announced that Fiennes would receive an honorary Doctorate of Science, from the University of Chester, in recognition of “outstanding and inspirational contribution to the field of exploration”.

In 2019, Fiennes appeared in a three part National Geographic documentary Egypt with the World’s Greatest Explorer (also titled Fiennes Return to Egypt) with his cousin and actor Joseph Fiennes that re-traced his first expedition in Egypt back in the 1960s.