The actor/director enjoyed a fling with Bonham Carter during their time filming 1994's Mary Shelley's Frankenstein and the romance led to the end of Branagh's six-year union to Thompson in 1995.
The Love Actually actress has remained quiet on the matter for almost two decades, but she now admits there is no bad feeling between her and Bonham Carter.
Speaking to Britain's The Sunday Times Style magazine, she says, "That is... blood under the bridge. You can't hold on to anything like that, I just think... pfft. It's pointless. I haven't got the energy for it. Helena and I made our peace years and years ago."
And Thompson jokes that her similarities to the younger star may have sparked Branagh's interest in Bonham Carter: "Oh, we are (similar). Being slightly mad and a bit fashion-challenged. Perhaps that's why Ken loved us both? She's a wonderful woman, Helena."
The trio have since appeared together in the Harry Potter film franchise.
Thompson is now married to Greg Wise, while Bonham Carter has two children with director Tim Burton. Branagh, who dated Bonham Carter for several years after his divorce, is now wed to art director Lindsay Brunnock.