Wylie described Cambridge Analytica as just one arm of a global company, SCL Group, that gets most of its income from military contracts but is also a political gun-for-hire, often in countries where democratic institutions are weak.
He suggested the company combines computer algorithms and dirty tricks to help candidates win regardless of the cost.
The 28-year-old Canadian says he helped set up Cambridge Analytica in 2013. He left the next year.
Wylie has previously alleged that Cambridge Analytica used personal data improperly collected from Facebook users to help Trump's 2016 presidential campaign.
Cambridge Analytica says none of the Facebook data was used in its work on the Trump campaign. It denies any wrongdoing.
Cambridge Analytica's acting CEO, Alexander Tayler, said that Wylie was a part-time contractor who "has no direct knowledge of our work or practices" since he left the company.
Wylie said he "absolutely" believes AggregateIQ drew on Cambridge Analytica's databases for its work on the Brexit campaign. In the closely fought referendum in 2016, 51.9 per cent of voters backed Britain's departure from the EU.
"I think it is incredibly reasonable to say that AIQ played a very significant role in Leave winning," Wylie said.
He testified that AggregateIQ was formed when Cambridge Analytica sought to expand but Canadians he wanted to bring into the business didn't want to relocate to Britain.
The two firms shared underlying technology and worked so closely together that Cambridge Analytica staff often referred to the Canadian firm as a "department," he said.
Because of the links between the two companies, Vote Leave got the "the next best thing" to Cambridge Analytica when it hired AggregateIQ, "a company that can do virtually everything that (Cambridge Analytica) can do but with a different billing name," Wylie said.
AggregateIQ, based in Victoria, British Columbia, issued a statement saying it has never been part of Cambridge Analytica or SCL.
"AggregateIQ works in full compliance within all legal and regulatory requirements in all jurisdictions where it operates," the company said. "All work AggregateIQ does for each client is kept separate from every other client."
Wylie's testimony came a day after Wylie and two other former insiders presented 50 pages of documents that they said proved Vote Leave violated election finance rules during the referendum campaign.
They allege that Vote Leave circumvented spending limits by donating £625,000 to the pro-Brexit student group BeLeave, which then sent the money directly to AggregateIQ.
Vote Leave denies breaking any campaign finance regulations. Dominic Cummings, the Vote Leave strategist, called Wylie a "fantasist-charlatan."
-AP
