Deadline Hollywood Daily is run by a former Newsweek and Los Angeles Times reporter, Nikki Finke, who has been described by the Financial Times as "the best connected reporter in the industry". It has been attracting 10m unique users per month.
Finke's scoops have included last week's news that Paramount had decided to part company with a senior executive, before the studio had told its own staff.
MMC has retained Finke, who also writes a Hollywood and media business column for LA Weekly, and allowed her to keep editorial control.
It is planning to invest in growing Deadline Hollywood Daily's editorial team and widening its distribution, which will pose a headache for movie industry trade publications Variety and The Hollywood Reporter and their websites.
MMC owns the Mail.com portal as well as a number of specialist sites publishing original content including celebrity and fashion-oriented HollywoodLife.com and automotive site OnCars.com. It is run by Jay Penske, who founded it in 2004.
Penske said: "Ever since the first scoop of Nikki's that I read, I quickly realised that she has raised the bar on -- if not changed the game of -- entertainment journalism.
"Her work perfectly embodies our mission at MMC -- to provide the most original and most authoritative content on the web."
The valuation of DHD is similar to the $10m CBS was reported to have paid in October 2007 for celebrity gossip blog Dotspotter.