The firm also said the new OS had reached the usage share milestone almost a year faster than its berated predecessor Vista.
But the growth of Windows 7 has yet to have an impact on Microsoft's overall share, which returned to its usual downward trend last month after a one-month advance.
Windows dropped to a 91.6 percent share, down half a percentage point from February.
The new OS again grabbed share from both Windows XP and Vista, with the former losing twice as much as the latter.
Windows XP slid to 64.5 percent, down a full point, while Vista lost 0.5 of a percentage point to end at 16 percent.
Windows Vista appears to be on a fast slide to nowhere: March was the fifth consecutive month that the beleaguered edition lost share, and the sixth month of the last seven in which it did so.
If Vista maintains its trajectory from the last three months, it will drop under 10 percent before the end of this year.
Windows 7 and Vista's paths should cross in June 2010, when the former is projected to become Microsoft 's second-most-used operating system.
Windows XP still accounts for the majority of the editions in use.
More than 70 percent of the computers running Windows in March relied on XP, which has three years of life left before Microsoft pulls the support plug in mid-April 2014.
Windows 7's gains show some signs of slowing, according to NetApplications. March's 1.5-point increase was slightly off the new operating system's monthly average gain of 1.7 points during the period from November 2009 to February 2010.
Even so, Windows 7 beat Vista to the 10 percent mark by nearly a year. While Windows 7 reached the milestone just five months after its public release, Vista didn't break the bar until May 2008, 16 months after its retail debut.
Meanwhile, Mac OS X's usage share rebounded by 0.3 of a percentage point last month to end at 5.3 percent.
The increase countered a recent downward drift by Apple 's operating system, which had lost share three out of the four previous months. March's increase was the largest for Mac OS X since December 2008.
But for Vince Vizzaccaro, executive vice president with NetApplications, the news last month wasn't the 10 percent for Windows 7, but the continued increase in online mobile devices.
By NetApplications' numbers, mobile operating systems powered 1.7 percent of the hardware used to access the web; that's an increase of 0.15 of a percentage point, and double the mobile share of August 2009.
"Mobile growth is now more a curve than a straight line," observed Vizzaccaro.
NetApplications measures operating system use by analysing the pool of about 160 million unique visitors each month to its clients' sites.
See also: Microsoft announces Windows 7 SP1