Qinetiq, once owned by the Ministry of Defence (MOD), said it would it pay $104.5m (£53m) in cash for the firm.
Dominion, a private company, employs 100 people, almost all of who all have high-level security clearance.
Qinetiq said the acquisition would allow it to operate in a market that is typically difficult to enter.
The deal is subject to US regulatory approval.
Qinetiq chief executive Graham Love said that the acquisition would help the firm expand in North America.
The MOD has been accused of selling off the firm to US private equity firm Carlyle in 2003 too cheaply.
The government retains a 19% stake in the firm.
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