The ban went into effect Thursday but police are not expected to issue a rash of fines as the public becomes accustomed to the regulation.
The National Police Chiefs' Council said in a statement that police would take an "educational, advisory and non-confrontational approach" for at least the first three months of the ban.
"This would see people being given warnings rather than being issued with fines," police said, according to an Associated Press report.
Smoking advocates called the new law unenforceable, but health officials and anti-smoking groups hailed the measure as the most important since a general ban on smoking in workplaces and enclosed public spaces took effect in England in 2007.
Penny Woods, chief of the British Lung Foundation, said the law brings England and Wales into line with other countries that have taken similar measures.
"Today is truly a cause for celebration for all those who care about protecting the health of generations to come," she said.
The ban means anyone who smokes inside a car with children under 18 inside will face a 50 pounds ($75) fine. Drivers face the same penalty even if they are not smoking.
The law applies even if vehicle windows or a sunroof are open. It does not apply to a convertible if the top is down and properly stowed — and use of electronic cigarettes is not affected by the legislation.
The ban has not been introduced in Scotland or Northern Ireland.