Fadi Fawaz said he made the terrible discovery after arriving at the musician’s home to wake him ahead of a planned Christmas Day lunch.
Mr Fawaz revealed his devastation as it emerged that Michael had been a secret donor to many charities, giving millions of pounds.
The man with a reputation for self-indulgence donated to charities involved with helping children, cancer victims and AIDS sufferers.
He was also remembered for other acts of kindness; volunteering at a homeless shelter and giving a stranger money for IVF treatment.
Mr Fawaz, who began seeing Michael following his split from Kenny Goss, said Michael had been looking forward to Christmas.
He told the Daily Telegraph: “I went round there to wake him up and he was just gone, lying peacefully in bed.
“We don’t know what happened yet.
“Everything had been very complicated recently, but George was looking forward to Christmas, and so was I.
“Now everything is ruined. I want people to remember him the way he was – he was a beautiful person.”
He also tweeted: “Its a xmas i will never forget finding your partner dead peacefully in bed first thing in the morning… I will never stop missing you xx.”
His former partner Mr Goss said he was devastated at the death, which is being treated by police as “unexplained but not suspicious”.
In a statement Mr Goss said: “I’m heartbroken with the news that my dear friend and long-time love George Michael has passed.
“He was a major part of my life and I loved him very, very much. He was an extremely kind and generous man.
“The beautiful memories and music he brought to the world will always be an important part of my life and those who also loved and admired him.”
Michael’s manager Michael Lippman said it was believed he had died from heart failure.
Details of Michael’s quiet acts of philanthropy emerged in the hours after his death.
Childline founder Esther Rantzen said Michael gave royalties from his 1996 hit “Jesus To A Child” to the charity.
“Over the years he gave us millions,” she said.
“We were planning next year, as part of our 30th anniversary celebrations to create… a big concert in tribute to him.”
:: Tributes to music legend
She said Michael was determined no-one outside the charity should know how much he gave to the nation’s most vulnerable children.
The singer – whose real name was Georgios Kyriacos Panayiotou – sold more than 100 million albums throughout his career.
His death came in a year which has seen the music industry lose some of its biggest stars including Prince, David Bowie and Leonard Cohen.