The funding round was led by GE/NBCU's Peacock Equity Fund, with a $6m investment, and additional funding from existing investors Clearstone Venture Partners and Mayfield Fund.
The size of the stakes now held by Peacock Equity Fund or Clearstone Venture Partners and Mayfield Fund were not disclosed.
Rubicon, launched in the US in 2007, aims to help advertisers and publishers profit from unsold advertising space by running technology to pull relevant ads from networks into the most appropriate spaces.
Among numerous high-profile executives, the Los Angeles-based firm counts Jay Stevens, the former MySpace European chief, as its vice-president and general manager of international.
Stevens was one of MySpace's first non-US executive appointments when he helped found MySpace UK in 2006.