This is quite interesting:
The tensions around Don McGahn’s exit and the Mueller investigation, explained
"McGahn was involved in at least four matters related to President Trump that Mueller is investigating."
"Just days after Trump became president, the FBI questioned his new national security adviser, Michael Flynn, about contacts he’d had with Russian Ambassador Sergey Kislyak during the transition. Flynn’s answers did not match what intelligence intercepts from surveillance of Kislyak showed — suggesting he’d lied to the FBI and perhaps to administration officials like Vice President Mike Pence as well.
Don McGahn was the first White House official to learn about this problem. Sally Yates — the Obama holdover serving as acting attorney general while Jeff Sessions was awaiting confirmation —
extensively briefed McGahn on Flynn’s interview, on January 26 and 27. McGahn then briefed other White House officials, including Trump, on what he’d learned.
Yet the White House took a curiously long time to act on this seemingly damning information. Eighteen days passed between McGahn’s first briefing and Flynn’s firing, and Mueller
reportedly wants to know why."
"...Sessions came under public pressure to recuse himself from any handling of the investigation into Russian interference.
But Trump very much did not want Sessions to recuse himself. So he told McGahn to tell Sessions that.
McGahn did so, urging the attorney general to remain in charge of the probe, according to the
New York Times. Sessions, however, recused himself anyway."
"...McGahn and his White House counsel’s office were also involved to some extent in deliberations over firing Comey — for instance, his team
researched the legality of the president firing the FBI director beforehand."
"...Finally, just about a month after Mueller was appointed special counsel, Trump reportedly tried to have him fired — and
told Don McGahnto make it happen. But according to
the New York Times, McGahn simply did not act on this presidential instruction, and Trump moved on."
And there's more about McGahn's role as White House Counsel. I think it's a good read.