Back then, Donald was her friend.

Hillary Clinton kicked off Father’s Day by posting a throwback photo of herself, a pre-presidential Bill, a pre-school-aged daughter Chelsea, and a lifesized Donald — Duck, not Trump.

The presumptive Democratic presidential nominee tweeted: “Happy Father’s Day, @BillClinton!”

But the snapshot, which appeared to have been taken in 1984 as Disney was celebrating the 50th anniversary of its iconic feathered character, and Bill Clinton was governor of Arkansas, drew immediate scorn from Clinton critics.

The first Twitter responder wrote, “WHERE’S MONICA?” in snarky reference to former White House paramour Monica Lewinsky.

The next Twitter user to respond said: “In the middle,” referring to the Donald Duck mascot.

The Post on Sunday exclusively excerpted former Bill Clinton Secret Service agent Gary J. Byrne’s tell-all — which details flings between the 42nd President and Eleanor Mondale, in addition to White House intern Monica Lewinsky.

Byrne, who was posted outside the Oval Office, told how he and other agents worked feverishly to keep Bubba’s fling with the late Mondale, daughter of former vice president Walter Mondale, from Lewinsky — and both of them from the first lady.

Excerpts of the tell-all, which appeared Sunday in The Post, described Mondale and Bill Clinton “in a compromising position, that is, making out on the Map Room table.”

Mondale had denied previous allegations of an extramarital fling, saying she and Bubba were “just friends.” She died in 2011 from brain cancer at the age of 51.

It’s been a busy weekend for the Clinton clan, as Chelsea gave birth to her second child with hubby Marc Mezvinsky on Saturday.

Hillary and Bill were spotted visiting their second grandchild, a boy, named Aidan, on Sunday at Lenox Hill Hospital on the Upper East Side.

Hillary clutched a cup of coffee and wore sunglasses as she entered with a team of Secret Service agents. A smiling Bill waved at well-wishers before getting to see grandchild No. 2 for the first time.

The presumptive Democratic nominee for president later tweeted out a photo of her and Bill holding Chelsea’s new baby, writing: “What a joy being with our new grandson, Aidan. So grateful. -H.”