According to comScore, Microsoft had eight per cent of the US smartphone market in the three months through January.
This is a drop of 1.7 percentage points.
The number one spot was claimed by Google's Android for the first time, climbing 7.7 percentage points to 31.2 per cent.
Research in Motion, meanwhile, fell 5.4 percentage points to 30.4 per cent.
Although Windows Phone 7 was launched by Microsoft in October, handsets loaded with the mobile operating system only went on sale in the US in November.
comScore's stats are therefore the first to cover sales of the platform.
Last week, wireless communications manufacturer Apple has launched the second generation of its iPad tablet computer at an event in San Francisco.
The machine features a faster processor, improved graphics and front and rear cameras, Apple said.
