In one of the paintings on display in this exhibition celebrating the 100th birthday of Derek Clarke, there is a sundial which reads: "How sweet is life, alas how swift." In talking to Clarke, who becomes a centenarian on Hogmanay, I felt this sentiment keenly.

The scene in Inverleith Park which he has depicted may have altered, but the sun still shines, and Clarke paints every day "so long as the light is good". His career has spanned 77 years. "That is why I am as old as I am," he tells anyone who looks taken aback at the sheer magnitude of his longevity.

Born on the last day of 1912 in Longthorpe, Peterborough, Clarke attended the famous Catholic boarding school, Ampleforth, before studying at London's Slade School of Art from 1931-1935. He just missed out on the era of the legendary Henry Tonks, widely regarded as the best and most formidable teacher of his generation, although Tonks's influence carried on at the college.

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Clarke's training in drawing and painting was rigorous and, after art college, he began to forge a reputation as a portrait painter. In 1936 he painted musician Elizabeth Poston, who had bought his painting The Nativity from the Royal Academy in 1935. Her introductions enabled him to travel to Ireland to paint portraits, and led to him spending a year painting in Rossaveel, Connemara, until the outbreak of war in 1939.

Today he says his experience in this remote corner of western Ireland, a peasant community he describes as "the heart and soul of the Gaeltacht", made him as an artist. But with war clouds looming, Clarke returned to London and joined the Durham Light Infantry the day after war was declared. He served in Tunisia and, in 1943, was shot in the spine.

"I should have been paralysed," he says, "but instead I got to have a whole year off and, for six months of that time, I painted. I still had all my pre-war colours of vermillion, viridian and cadmium, influenced by Van Gogh and Cezanne while everyone else was painting in the drab colours available at time."

During this time, he won admiration from acclaimed artist Stanley Spencer for work he showed at the New English Art Club. Then, as a newly married man, he went back to western Ireland when the war ended. After his wife Ann, then pregnant with the eldest of their four sons, returned home to her mother in England, he was able to immerse himself in his work. While many artists at the time turned to abstraction and surrealism, Clarke felt the need to return to nature and "start learning again".

Learning and teaching are one and the same for Clarke, who was asked to join the staff of Edinburgh College of Art (ECA) in 1947 by the principal, Robert Lyon, who was keen to introduce new teaching ideas. In this febrile post-war atmosphere, many of the students were, recently demobbed from the armed services, like Clarke, while the rest were fresh from school. The young tutor's colleagues included Robin Philipson, Alan Carr, Leonard Rosoman (a fellow Englishman who taught mural painting), Johnny Maxwell and William Gillies, the head of school.

His attitude to drawing as a tool for information-gathering and exploring the visual world was admired, and he went on to introduce the teaching of human and comparative anatomy.

The list of Clarke's former students reads like a who's who of Scottish art and includes Elizabeth Blackadder and her husband John Huston, as well as John Bellany, Barbara Rae, Victoria Crowe and George Donald. Crowe and Donald have helped to put together this exhibition of their former teacher and colleague's work, which includes works from both private collections and the artist's studio.

This exhibition presents an astonishing body of work by a man who was made an Associate of the RSA in 1989 and, in 2005, attained full Academician status. The earliest work on show is a self-portrait in pen and ink from 1935, while the most recently completed is Pat And I At Albanich II, an oil painting made between 1994 and 2012, depicting the artist and his wife locked in an embrace on a grassy plane near Merkland, Sutherland. With typical brio, he says of it: "I climbed up to the scene of this painting for three consecutive days, lashed the easel and canvas, and then myself, to the trees, before starting work."

There is a lyrical feel to much of Clarke's later paintings. Two struck me as intensely moving: Picnic By The North Tyne (1959-60/2003) and Perro At Humshaugh (1960). Both depict snapshots of family life, with Clarke's second son, Peregrine, in the limelight in both. Standing in front of the paintings, Clarke told me he returned to Picnic At North Tyne – a portrait of his family in his late wife's native Northumberland – in 2003, after a 44-year gap, when his son was dying of cancer. "I was painting him alive in the morning and watching him die in the afternoon," Clarke states simply.

This painting of a naked yet strangely non-vulnerable boy, with spun gold hair, staring life in the face, is mesmerising, as is Perro At Humshaugh, in which the boy stands nonchalantly on one foot beside a window and a vase of flowers, the light streaming in on him.

For someone drenched in the art of painting and whose work is so pure, it was fascinating to hear Clarke then say that in 50 years' time he suspects "painting will have played itself out... It's already happening and is the way of things. Creative people will find avenues for their creativity."

Go see this exhibition. I'll be back.

Derek Clarke RSA: The Artist At 100 is at RSA Projects, Royal Scottish Academy, The Mound, Edinburgh (0131 225 6671, www.royalscottish, from January 1-31