The Oxford English Dictionary confirms how I view the difference, which is that a memoir is generally not considered a full historical record of a life, while an autobiography or a biography is thoroughly researched.
My Simple Explanation: A memoir is written in first person (“I”) and includes only information that the subject of the memoir has told the author. A biography is written in third person (“he/she”) and puts into full context the events of the person’s life; therefore a biography includes 30 to 70 percent material that has been researched and is not known to the subject of the biography.
Even Simpler Explanation: A memoir is “my story, as I told it.” No one expects a memoir to be researched.
An advantage of the biography is that it offers a full contextual narrative that reads like a novel but is non-fiction.
An advantage of the memoir is that it gives a full immersion into the subject’s voice.
Most of our memoirs have been published for family members only, but one sample of one of our memoirs, Helene Pittler’s My Lives, is here on Amazon.com, where you can view and search inside.