"Google is a monopolist, and it has acted as one to maintain its monopoly," Mehta wrote in his ruling.
The findings, if upheld, could outlaw contracts that for years all but assured Google's dominance.
Google said it planned to appeal the decision, which it said "recognizes that Google offers the best search engine, but concludes that we shouldn’t be allowed to make it easily available."
Judge Mehta ruled that Google violated antitrust law in the markets for "general search" and "general search text" ads, which are the ads that appear at the top of the search results page.
He said Google was not liable in the market for "search advertising" because it did not hold a monopoly there.
The decision is a huge win for the Justice Department and could have giant implications for some of the other big names in the tech world.