Scientists reported that sensors on their drill system had noted a change in pressure, indicating contact had been made with the lake.
A camera was then sent down to verify the breakthrough, 'BBC News' reported.
The Whillans project is one of a number of such ventures trying to investigate Antarctica's buried lakes.
In December, a British team had abandoned its efforts to get into Lake Ellsworth after encountering technical difficulties.
The Russians have taken water samples from Lake Vostok, although they have yet to report any big discoveries, the report said.
Lake Whillans is situated in the west of Antarctica, on the southeastern edge of the Ross Sea. It is less of a lake and more of a dense system of streams, almost like a delta, that covers some 60 square kilometre. The liquid body is quite shallow - just a few metres in depth.
The Whillans Ice Stream Subglacial Access Research Drilling (Wissard) team has been using a hot-water drill to melt a 30cm-diameter hole through the overlying ice.
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The intention, now that the hole is secure, is to lower various sampling tools and sensors into the lake to study its properties and environment.
Samples will be assessed onsite at the ice surface in temporary labs, and others will be returned to partner universities for more extensive analysis.
The thickness of the overlying ice was measured to be 801m, which agreed well with the estimates from seismic imaging, the Wissard blog said.
More than 300 large bodies of water have now been identified under the White Continent.