The study also found that by one year of age, one in seven toddlers is using devices for at least an hour a day.
Researchers developed a 20-item survey to find out when young children are first exposed to mobile media and how they use devices.
Parents of children ages 6 months to 4 years old who were at a hospital-based paediatric clinic that serves a low-income, minority community were recruited to fill out the survey.
Participants were asked about what types of media devices they have in their household, children's age at initial exposure to mobile media, frequency of use, types of activities and if their pediatrician had discussed media use with them.
Results from 370 parents showed that 74 per cent were African-American, 14 per cent were Hispanic and 13 per cent had less than a high school education.
Media devices were ubiquitous, with 97 per cent having TVs, 83 per cent having tablets, 77 per cent having smartphones and 59 per cent having Internet access.
Children younger than one year of age were exposed to media devices in surprisingly large numbers: 52 per cent had watched TV shows, 36 per cent had touched or scrolled a screen, 24 per cent had called someone, 15 per cent used apps and 12 per cent played video games.
By 2 years of age, most children were using mobile devices.
"We didn't expect children were using the devices from the age of 6 months," lead author Hilda Kabali, a third-year resident in the Pediatrics Department at Einstein Healthcare Network, said.
"Some children were on the screen for as long as 30 minutes," she said.
Results also showed 73 per cent of parents let their children play with mobile devices while doing household chores, 60 per cent while running errands, 65 per cent to calm a child and 29 per cent to put a child to sleep.
Time spent on devices increased with age, with 26 per cent of 2-year-olds and 38 per cent of 4-year-olds using devices for at least an hour a day.
Finally, only 30 per cent of parents said their child's pediatrician had discussed media use with them.
The study was presented at the Pediatric Academic Societies (PAS) annual meeting in San Diego.