In an interview with The Sunday Times, the BBC’s former political editor said there was a “decent argument” that Mr Johnson’s involvement in the Brexit debate “tipped” the balance towards leaving the EU.
She said there was also a case to be made that should Mr Johnson not have won the Tory leadership contest in 2019, Britain may never have made its exit.
Asked how he will be remembered, she said: “He will, without doubt, always be seen as a prime minister of huge consequence.
“Because whatever you think of the UK decision to leave the EU, there’s a decent argument to be made that his involvement tipped [it], but there’s also a decent argument that if he hadn’t won the Tory leadership in 2019, it [Brexit] wouldn’t have happened.
“So he will have a chunky chapter in UK history. Then being the prime minister who was in charge during the pandemic, not least the PM who nearly lost his life to the disease.”
Ms Kuenssberg also described Mr Johnson’s likely successor, Liz Truss, as a “great survivor”.
“She has often been looked down on by people who she then outwitted or outlasted,” she said.
“She is a great survivor. Relentless, shape-shifting, being pragmatic, having a bit of fun at her own expense – those are all things that she is [or is] willing to do. She’s instinctive.”
On her own next steps, she said she wants her version of the Sunday programme to have some “wit around it and warmth”.
Ms Kuenssberg said there is no point “being aggressive for aggressive’s sake”.
“But firm? Absolutely damn right,” she said.
Ms Kuenssberg is due to take over the Sunday politics show on September 4 with a new set, format and title music.