The micro mutt who lives with her Argentine owners in the north east Puerto Rican town of Vega Alta, was so small when she was born back in December 2011 she had to be nourished with an eye dropper and could fit in a teaspoon.
And regardless of being fed almost entirely on gourmet food, she failed to grow much larger than the size of an ordinary Chihuahua puppy, now weighing around a pound.
Milly is always accompanied by Ms Eizzil or by her owner, Argentinian Vanessa Semler, who is dedicated to rearing chihuahuas in Puerto Rico.
Ms Semler says caring for Milly takes effort, but explains that she and her husband, Edwin Davila, have no children so they have the time to focus their energies on their pet project.
Delighted owner Vanessa Semler said: 'We have to be certainly careful with Milly because she is too small so she can be easily lost.
'When people see Milly they are really surprised because she is so small but they really love her because she is so lovable.' Milly, who sleeps in a baby's crib, beat previous title-holder Boo Boo, a long-haired female Chihuahua, owned by Lana Elswick of Raceland, Kentucky, who at 4.2inches towers over the new record holder.
Pocket-sized pets remain one of the most fascinating areas of the Guinness World Records database, with tiny dogs arguably the most popular of all our animal records.
Boo Boo, a long-haired, female chihuahua was the former holder of the title of World's Smallest Dog. She measures 10.16cm tall and is owned by Lana Elswick of Raceland, Kentucky
Born in December 2011, she weighs roughly 1 pound (half a kilogram) and is known for often sticking out her tiny tongue when someone takes her picture. 'She knows how to pose,' says Vanesa.
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