The annual survey by the Consumer Reports found that thefts were largely responsible for the increase in devices that were permanently separated from their owners.
The report said stolen devices nearly doubled from 1.6 million in 2012 to 3.1 million in 2013.
The number of lost devices increased from 1.2 million in 2012 to 1.4 million in 2013.
More than 3,100 adults in the US participated in the Consumer Reports survey that was used to calculate the figures.
A vast majority of smartphone owners (34 per cent) neglected to take more aggressive measures, such as creating longer passcodes and installing software that could locate their phone or remotely erase its contents, the report said.
Consumer Reports recommends that at the minimum consumers secure their smartphones by setting up a four-digit lock code.
"Given how much personal information these devices containfrom photos, contacts, and e-mail accounts to social-networks, shopping, and banking apps - it's understandable that you'd freak out if either misfortune happened to you," said the report.
Still, there are steps people can take on any phone to guard against thieves.
But many users are still not protecting themselves sufficiently, the survey found.
About one-third of the smartphone owners surveyed said that they weren't taking even the simplest measures to protect their phone and the data on it.