Lack, the son of a well-known and prosperous London surgeon, was educated at Cambridge University. From 1933 to 1940 he taught at Dartington Hall, a progressive private school, except for a year's leave in 1938 spent studying the birds of the Galapagos. During World War II he served with the Army Operational Research Group, helping to develop radar. This experience was later applied when he used radar in his studies of bird migration. In 1945 he finally became a professional ornithologist and served as director of the Edward Grey Institute of Field Ornithology, Oxford, until his death.

Lack's first substantial work, the widely read Life of the Robin (1943), was followed by the publication of his Galapagos material in Darwin's Finches (1947), a fascinating account of the 14 specialized species of finch that have evolved from an original invading flock of ordinary seed-eating finches. Such fieldwork inevitably led Lack to the consideration of more theoretical questions.

In particular, he studied the factors controlling numbers in natural populations and concluded that such factors act more severely when numbers are high than when they are low. The irregularities of population fluctuation suggested to Lack that the control mechanisms must be very complex. His data and theories are discussed in such works as Natural Regulation of Animal Numbers (1954) and Population Studies of Birds (1966).

Such data was variously interpreted, for instance by Richard Dawkins, who claimed that it supported the theory of the ‘selfish gene’; Lack himself was more inclined to use the data to support the idea of group selection.–