He was educated at Robert Gordon"s College in Aberdeen. He then attended both Edinburgh and Oxford University training as a botanist, specialising in trees, receiving a postgraduate doctorate (Doctor of Science) from Oxford in 1927.
He is especially remembered for the recording and classification of trees on the Indian sub-continent. He was also an expert on Spermatophytes. From 1927 to 1929 he worked with the Indian Forest Service and conducted many studies of tree species throughout India.
In February 1929 he made a study trip around the Near East with fellow botanist Cyril Darlington.
He worked at the Royal Botanic Garden Edinburgh 1930 to 1954 alongside Roland Edgar Cooper (both then under William Wright Smith). In 1954 Cowan took over as Curator of Inverewe Garden on the west coast of Scotland.
During the Second World War he provided valuable advice to the Ministry of Supply in relation to the Home-Grown Timber Production Department. In 1931 he was elected a Fellow of the Royal Society of Edinburgh his proposers including Sir William Wright Smith, William Grant Craib and Albert William Borthwick.
He served as President of the Botanical Society of Edinburgh 1951-1953.
Queen Elizabeth created him a Commander of the British Empire (Commander of the Order of the British Empire) in 1952. He died in 1960.
Their daughter was Pauline Cowan, later Professor Pauline Harrison (b1926).