Tom Hanks, Jay Leno, Holly Robinson Peete, Loretta Devine and Stevie Wonder were among the celebrities who helped celebrate Duncan's life in the nearly four-hour service.
"Just to see such a pure heart and pure kindness, and to see it taken so early," the comedian said, his voice cracking. "There are no sadder words than what might have been."
He said that Duncan's ever-present smile "grew another millimeter on each end" when he met his fiancee, Omarosa Manigault. The two had planned to marry in December.
Tom Hanks told a story about how a young Duncan growing up on the South Side of Chicago joined a gang, only to get whacked upside the head with a frying pan by his mother.
He then had to sheepishly tell the gang that this mother wouldn't let him, Hanks related.
He also teased about Duncan's penmanship, saying it was "loopy and large" and looked "like a 12-year-old girl who loves horses wrote it."
Reverend Dr. H. Beecher Hicks delivered the final eulogy, saying to Duncan's fiance, Omarosa Manigault, "Don't fret, he's in heaven."
A large crowd turned out Sunday for a public viewing of Duncan's casket at the Hall of Liberty in Forest Lawn.
There was also a viewing on a screen of "Green Mile," the film that made Duncan a super star.
Duncan died at Cedars-Sinai Medical Center on Sept. 3 after nearly two months of treatment after a heart attack. He was 54 years old.
Duncan was hospitalized after suffering a myocardial infarction on July 13.
He was moved from the intensive care unit nearly a month later in August, but remained in the hospital.
Coroner's officials said there was nothing to suggest officials needed to review the case.
Officials noted that Duncan had heart problems and was under medical care.
In such cases, it is normal for the doctor to sign the death certificate without any review by the coroner.
In the days since Duncan's death, a growing memorial with flowers, candles, notes and cards has appeared outside his Woodland Hills home.
Friends say that the man with a big smile and an even bigger heart was unaffected by his success, and never let fame go to his head.