Morgan, who was also a judge on "America's Got Talent," served as editor at News of the World in 1994 and 1995, before helming the Mirror, where he stayed until 2004.
During a July Parliamentary hearing with Rupert Murdoch and his son James Murdoch, committee member Louise Mensch accused Morgan of publishing an article in 2002 that had been obtained via phone hacking.
Morgan denied the accusation and demanded an apology from Mensch.
The former Fleet Street editor also fought off accusations from James Hipwell, a former Daily Mirror financial columnist who called illegal phone hacking "endemic" during Morgan's tenure. Hipwell is scheduled to testify Wednesday before the inquiry.
"Piers was extremely hands-on as an editor," Hipwell told British newspaper The Independent in July. "I can't say 100 percent that he knew about it. But it was inconceivable he didn't."
A spokesman for Trinity Mirror, the publisher of The Daily Mirror, denied Hipwell's allegations to The New York Times, but has the Mirror launched its own investigation.
Morgan also challenged Hipwell's credibility to the New York Times, pointing out that Hipwell served a brief stint in jail for profiting on stocks he touted in his column.
Morgan has his defenders. He told The New York Times that he was grateful to see a tweet from Tom Watson, another member of the Parliamentary committee questioning the Murdochs.
"I've not seen any evidence linking @PiersMorgan to hacking. And I've seen a lot of documentation these last 2 years," Watson wrote.