From May 2014, anyone wishing to buy cigarettes in the city will be required to show ID proving they are 21 or over, making it the first large city in the United States to raise the legal age of purchase so high.
Bloomberg said the law, which was passed by city councilors on October 30, "will prevent young people from experimenting with tobacco when they are most likely to become addicted."
The new law reinforces New York City's already stringent laws against smoking, which is forbidden in bars, restaurants, parks, public spaces and city beaches.
Across the rest of New York state, with a total of almost 20 million people, the age for purchasing cigarettes remains 18.
Taxes on cigarettes in New York City are also the highest in the United States at $5.85 per pack, which sets the price of a pack at around about $12 (8.87 euros).
Bloomberg also signed another law on Tuesday which limits the cost of cut-price cigarettes sold with special discounts so that even the least pricey pack will still cost $10.50 dollars.
The effects of New York City's tough anti-smoking stance has seen declining numbers of adult smokers, slipping from 21.5 percent in 2002 to 14.8 percent in 2011, according to city hall. However the rate among young smokers has remained steady at around 8.5 percent since 2007, prompting the latest law.
Authorities hope the legislation will reduce smoking between 18 and 20-year-olds by as much as 55 percent.
Most cities and regions in the United States set the legal age for cigarette and tobacco sales at 18, although some states have raised the threshold to 19.
And the small town of Needham outside Boston beat New York City to the punch, raising the age to 21 in 2005.