In her testimony, she detailed a July 10 meeting she attended with senior Ukrainian officials, then-National Security Adviser John Bolton, and other US officials in which the American ambassador to the European Union, Gordon Sondland, raised the issue of the investigations, according to informed sources.
People in the room took the comments to refer to an investigation that could implicate Biden and his son Hunter, the sources said.
Both Hill and Bolton left the meeting with concerns about what had transpired, and Hill said Bolton instructed her to talk to NSC lawyer John Eisenberg.
A Ukrainian gas company paid Hunter Biden USD50,000 a month to sit on the board at a time when his father, president Barack Obama's vice president, was pushing anti-corruption efforts in Ukraine.
Trump and his allies have described that as a corrupt arrangement. Ukrainian officials have produced no evidence of wrongdoing by either Biden, and they both deny they did anything wrong.
Sondland also appeared to be coordinating his efforts with acting White House chief of staff Mick Mulvaney, Hill testified on Monday.
The envoy is set to testify before House committees on Thursday.
Hill is the latest administration official to testify as part of the impeachment inquiry, following hearings with Marie Yovanovitch, the former US ambassador to Ukraine, and Kurt Volker, the former American envoy for Kiev negotiations.
House Intelligence Committee chairman Adam Schiff on Monday called Hill "courageous" for her testimony and said of her and Yovanovitch "These are two of the most impressive women to testify in front of Congress."
In addition to Sondland's testimony, House committees will hear this week from two more key witnesses. Michael McKinley, a top aide to Secretary of State Mike Pompeo who resigned last week will testify on Wednesday.
Laura Cooper, the Deputy Assistant Secretary of Defence overseeing Ukraine, will testify on Friday.
Democrats have alleged that Trump's efforts to get Ukraine to launch the investigations marked an improper use of the President's office to benefit his own re-election hopes, resulting in the launch of the impeachment inquiry.
Trump has denied wrongdoing and on Twitter Monday morning called the Democrats' probe a "total impeachment scam".