Mr Al Duwaisan, 75, the longest-serving ambassador to Britain, said he loved the queen for her energy, knowledge and sense of humour.
Speaking from his home in Kuwait on Friday, the recently retired Mr Al Duwaisan said he met the monarch on more than 150 occasions, with the final time being for her Platinum Jubilee in June.
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Reflecting on this encounter, he said: “I can’t believe that at this age, her mind, her body, her sense of humour – she was wonderful.“
While they didn't meet again, the following month Mr Al Duwaisan was appointed to the Order of St Michael and St George, an honour bestowed by the queen and reserved for those who “render extraordinary or important non-military service in a foreign country”.
And it wasn't the first time the queen had honoured the man who joined Kuwait’s Ministry of Foreign Affairs in 1970.
In 1995, she made him an Honorary Knight Grand Cross of the Royal Victorian Order. He became Dean of the Diplomatic Corps in 2003, a role which has seen him advise more than 750 newly-arrived heads of mission about diplomatic life in London.
Mr Al Duwaisan recalled their first meeting in 1993 when he was only months into the job as a London-based ambassador.
Due to royal protocol, his diplomats had to ensure they never turned their back to the queen, which involved them walking backwards while wearing a traditional men’s cloak, known as a Bisht.
“I had about seven diplomats with me, and all of them were wearing the robe – the big robe taller than them – because this is our system," Mr Al Duwaisan said.
“I was afraid that if they stepped backwards they would fall, and it would be embarrassing."
The ambassador said that after some discussion, he found a way for the men to manoeuvre safely around the queen.
“She liked it,” he said.
Mr Al Duwaisan was later invited for a dinner at Windsor Castle with his wife, where he sat beside the queen.
“I will never forget that she had a system in the past, to invite the ambassador to stay in Windsor Castle, so I went with my wife at 5pm at the reception and private dinner,” he said.
“I was on her right. So we talk about a lot of things, of course they told me not to talk about politics.
“We had a long conversation about her children, going to Kuwait – and the Kuwaitis who are coming to England.
“I said to her, that’s because the relation between our two countries is very close – very, very close.”
The queen showed Mr Al Duwaisan a large sword given to King George V by Sheikh Ahmed Al Jaber, whose son is now the Emir of Kuwait.
“I feel sad now to remember all this, but I say to myself, this is life, she lived a good life, she lived a long life," the diplomat said.
“Britain will not be the same as when she was in power.”
Mr Al Duwaisan said the queen’s death has been felt deeply in Kuwait, with many official buildings bearing the British flag to mark her passing. He said the monarch provided a sense of stability and fostered an important relationship between the UK and Kuwait during years of turmoil in the country, including through the Iraq invasion in 1990.