That figure from April is an increase from the 48.6 percent at the start of the year, the report of mobile market data from comScore said.
In April, Apple's iOS claimed 31.4 percent of the market, trailed by Research in Motion's BlackBerry at 11.6 percent, Microsoft's Windows Mobile/Windows Phone at 4 percent and Symbian at 1.3 percent, mediapost.com reported.
The April survey found more than 107 million Americans owned smartphones, a 6 percent increase from January.
About half of users are both downloading apps and using mobile browsers, while more than a third are going to social networking sites or blogs, a third play games, and a quarter listen to music on their devices, the comScore survey found.
Text messaging remains the most popular non-voice mobile usage, with some 74 percent of smartphone owners texting, comScore said.