Jayal began his mountaineering career while he was a student at The Doon School, where his teachers encouraged his interest in climbing as a way to tame his somewhat unruly nature.
Jayal's first major expedition as a 16-year-old schoolboy was to the Awar Valley above Badrinath, reaching 6,000 meters. Other climbs, while still at Doon, included Trisul with Gurdial Singh, a teacher from Doon. While Singh went on to reach the peak of Trisul, where he performed a headstand asana to honor the Hindu god Shiva, Jayal noted his own feelings in lyrical terms: "The grass on which we camped was like a cushion sprinkled with tiny mauve primula and the gentle lapping of the running water recalled melodies from Beethoven’s Pastoral Symphony. I confess a desire to bring my efforts to an honourable conclusion here – as long as somebody got to the top – and revel in this bracing and saner altitude."
It was a remarkable transformation of Doon's most delinquent boy who had become a "gentle, perfect knight"
In 1948, Nandu Jayal went to Switzerland and acquired a Ski Teacher's Certificate, a very respectable achievement. He was appointed Chief Instructor at the Winter Warfare School, later known as the High Altitude and Winter Warfare School.
Under the leadership of the Engineer-in-Chief, Maj. Gen. Harold 'Bill' Williams, himself an eager climber, Nandu Jayal organised the first Sappers expedition to Bandarpunch successfully in 1950. As a young Captain in 1950-51, he carried out a strategic reconnaissance of the Garhwal Himalayas and was later the Indian Army liaison officer for the French Expedition to Nanda Devi in 1951. He organised and led two expeditions to Kamet; the first in 1952 when the summit team was forced back by a blizzard from just 600 metres short of the mountain and later in 1955 when he summitted - at that point of time, it was the highest that an Indian had climbed.
Major Jayal was the founder principal of the Himalayan Mountaineering Institute at Darjeeling, with Tenzing Norgay of Everest fame as the Chief Instructor. Both of them were invited by the Swiss Government to Switzerland where they spent three months seeing new things and having new experiences. Maj Jayal became the only non-Swiss to win the coveted Swiss Guide's Diploma and Badge. Jayal led the 1955 Kamet expedition as the Director HMI.
Maj Jayal organised an expedition to Nanda Devi in 1957. Bad weather thwarted the expedition but Jayal, never one to give up, went next to the Karakoram where he conquered Saken (24,130 ft) and Sakang (24,150 ft), the third highest peak in the Karakoram range.
In 1958, the Government sponsored an expedition to Cho Oyu (28,867 ft), one of the highest mountains of the world. Jayal died of pulmonary oedema caused by overexertion on this expedition at Camp I. He had started late and tried to catch up with the main party. There was also a problem in his medical care as much of the expedition equipment had been lost in a Dakota crash en route to Nepal. His death and that of some others brought home the cruel lesson of need for acclimatisation and discipline in the pursuit of Himalayan mountaineering.
Pandit Jawaharlal Nehru, the Prime Minister of India, paid rich tributes to Jayal saying "the Major has set an example of courage and adventure which should inspire our young people. The news of his death came to me as a shock and I feel that the country has suffered the loss of her finest mountaineer..."
Arthur Foot, Jayal's Headmaster of Doon, noted that "The Himalaya completed his education into a stature of nobility", echoing a sentiment expressed two years earlier by Jayal himself who had noted, after an expedition to Saser Kangri, that "Pushing the body to the utmost for something indefinably inherent in a person, is intrinsically noble and worthwhile." R.L. Holdsworth, a teacher at Doon who had encouraged Jayal to pursue mountaineering noted after his death that "He died very much the master of himself and of most of the world that is worth mastering."
The Indian Mountaineering Foundation had a Nandu Jayal Fund and published, along with the Corps of Engineers, a book Nandu Jayal and Indian Mountaineering, which contained articles on various aspects of Indian Mountaineering by him and by others.
Nandu Jayal's life and career motivated many young officers of the Corps to take up mountaineering, most prominent of whom were his nephews, Harsh and Jai Bahugana; both were officers of the Indian Army, dedicated mountaineers and both died on Mount Everest.