Lamb graduated in 1935 and, after working at the Meteorological Office, London, where he led the climatic variation research, he started the Climate Research Unit at the University of East Anglia, Norwich. He eventually became honorary professor in the School of Environmental Science at that university.

His main aim was to build up a detailed picture of the climates of the past and to acquire sufficient understanding of them to be able to see how climate might develop. By examining such records as ships' logs he was able to reconstruct much of the climate of the last two to three hundred years; for earlier periods he used such evidence as the fluctuations in tree-ring width. The results of his researches were published in his two-volume Climate: Present, Past and Future (1972, 1977).

To understand such major climatic changes as the ‘Little Ice Age’ (1550–1850) or the unusually warm weather of the 1900–50 period Lamb has searched for possible physical mechanisms. To investigate the effects of volcanic dust, for example, he introduced the Dust Veil Index, which measures the amount of dust released each year on a scale that assigns the arbitrary value 1000 to the great Krakatoa eruption of 1883. As the present cooling has begun in a period with a low index it cannot, Lamb argued, be the crucial factor.