His father was a Communist businessman of Russian ancestry, while his Jewish mother was born in Germany and came to the United Kingdom as a refugee from the Nazi regime.

In 2005 he retired early as a consultant psychiatrist, writing in the Sunday Telegraph: "Retired at last! Retired at last! Thank God Almighty, retired at last! Such are the feelings of almost all hospital consultants and general practitioners who retire from the National Health Service after many years of service: years that increasingly have been ones of drudgery, servitude and subordination to politicians and their henchmen, the managers, who utter Pecksniffian pieties as they secure the advancement of their own inglorious careers." He now divides his time (with his wife, Agnes) between homes in Bridgnorth, Shropshire, and France, and continues to write.

He has worked in the present Zimbabwe (then Southern Rhodesia), Tanzania, South Africa, Kiribati, the east end of London and central Birmingham (UK), amongst other places.

Regarding his pseudonym Theodore Dalrymple, Daniels says he "chose a name that sounded suitably dyspeptic, that of a gouty old man looking out of the window of his London club, port in hand, lamenting the degenerating state of the world."

He is an atheist, but he has criticized anti-theism and says that "to regret religion ... is to regret our civilization and its monuments, its achievements, and its legacy". Although raised as a Christian, he began doubting the existence of a God at age nine. He became an atheist at about age fourteen in response to a moment in a school assembly.

Daniels has also used the pen names Edward Theberton and Thursday Msigwa and possibly yet another pen name.

Daniels has written extensively on culture, art, politics, education, and medicine drawing upon his experience as a doctor and psychiatrist in Zimbabwe and Tanzania, and more recently at a prison and a public hospital in Birmingham, in central England.

His work frequently appears in The British Medical Journal, The Times, The Observer, The Daily Telegraph, The Spectator , The Salisbury Review and in American and other foreign publications, including "Axess" (a Swedish cultural magazine), and City Journal, a magazine published by the Manhattan Institute.

In 2009, Dalrymple's British publisher Monday Books announced it was to publish two books. The first, Not With a Bang But A Whimper, appeared in August 2009. It is different from the US book of the same name, though some of the author's essays appear in both books. In October 2009, Monday Books was to publish Second Opinion, a further collection of Dalrymple essays, this time dealing exclusively with his work in a British hospital and prison.

In his commentary, Daniels frequently argues that the so-called "progressive" views prevalent within Western intellectual circles minimize the responsibility of individuals for their own actions and undermine traditional mores, contributing to the formation within rich countries of an underclass afflicted by endemic violence, criminality, sexually transmitted diseases, welfare dependency, and drug abuse.

He contends that the middle class abandonment of traditional cultural and behavioural aspirations has, by example, fostered routine incivility and ignorance among the poor. Although he is occasionally accused of being a pessimist and misanthrope, his defenders praise his persistently conservative philosophy, which they describe as being anti-ideological, sceptical, rational and empiricist.

Daniels' writing has some recurring themes.

• The cause of much contemporary misery in Western countries - criminality, domestic violence, drug addiction, aggressive youths, hooliganism, broken families - is the nihilistic, decadent and/or self-destructive behaviour of people who do not know how to live. Both the smoothing over of this behaviour, and the medicalization of the problems that emerge as a corollary of this behaviour, are forms of indifference. Someone has to tell those people, patiently and with understanding for the particulars of the case, that they have to live differently.
• Poverty does not explain aggressive, criminal and self-destructive behaviour. In an African slum you will find among the very poor, living in dreadful circumstances, dignity and decency in abundance, which are painfully lacking in an average English suburb, although its inhabitants are much wealthier.
• An attitude characterized by 'gratefulness' and 'obligations towards others' has been replaced, with awful consequences, by an awareness of rights, a sense of entitlement. The result is resentment as, naturally, those rights are violated by parents, authorities, bureaucracies and others in general.
• One of the things that makes Islam attractive to young westernized Muslim men is the opportunity it gives them to dominate women.
• Technocratic or bureaucratic solutions to the problems of mankind produce disasters in cases where the nature of man is the root cause of those problems.
• It is a myth, when going "cold turkey" from an opiate such as heroin, that the withdrawal symptoms are virtually unbearable; they are rarely worse than flu.
• Criminality is much more often the cause of drug addiction than its consequence.
• High culture and refined aesthetic tastes are worth defending, and despite the protestations of non-judgmentalists who say all expression is equal, they are superior to popular culture.
• The ideology of the welfare state is used to diminish personal responsibility. Erosion of personal responsibility makes people dependent on institutions and favours the existence of a threatening and vulnerable underclass.
• Moral relativism can easily be a trick of an egotistical mind to silence the voice of conscience.
• Multiculturalism and cultural relativism are at odds with common sense and statistical evidence.
• The decline of civilised behaviour--such as: self-restraint, modesty, zeal, humility, irony, detachment--is a disaster for social and personal life.
• The root cause of our contemporary cultural poverty is intellectual dishonesty. First, the intellectuals have destroyed the foundation of culture, and second, they refuse to acknowledge it by resorting to the caves of political correctness.
• Beyond and above all other nations in the world, Britain is the place where all the evils summarized above are most clearly manifest.