Mandino was born December 12, 1923. According to the 1930 U.S. Census, he was named after his paternal grandfather.

Mandino was once the editor of a high school paper and planned to attend the University of Missouri's journalism school. But in the summer of 1940, before Mandino was able to enter college, his mother died suddenly from a massive heart attack. This happened while Mandino's mother was in the kitchen preparing his lunch. After the incident, Mandino decided to work in a paper factory until 1942. Afterwards, Mandino joined the United States Army Air Corps where he became a military officer and a bombardier. He flew for thirty bombing missions over Germany on board a B-24 Liberator during World War II. It was also during this time that Mandino flew with fellow pilot and movie star, James Stewart. Mandino kept a personalized 8 X 10 photograph of Stewart. This photograph hung on the wall of his home office where he wrote his books.

After his military duties, Mandino discovered that many companies were not hiring many former bomber pilots. As a result, he became an insurance salesman. Traveling on the road and sitting in bars at night, Mandino became an alcoholic. He was unable to keep a job. As a result, Mandino's wife, together with their only child, left him. One wintry November morning in Cleveland, Mandino almost tried to commit suicide. But as he sorted through several books in a library, volumes of self-help, success and motivation books captured Mandino's attention. He selected some titles, went to a table and began reading. Mandino followed his visit to the library with more visits to many other libraries around the United States. He read hundreds of books that dealt with success, a pastime that helped him alleviate his alcoholism. It was in a library in Concord, New Hampshire, where he found W. Clement Stone's classic, Success Through a Positive Mental Attitude, a book that changed Mandino for the better.

Mandino eventually became a successful writer. His works were inspired by the Bible and influenced by Napoleon Hill, W. Clement Stone, and Emmet Fox.

Mandino wrote The Greatest Salesman in the World which contains the "time-tested wisdom of the ancients distilled into ten simple scrolls" which, if followed for the prescribed ten months, will as Og says, "will seep into my other mind, that mysterious source which never sleeps, which creates my dreams, and often makes me act in ways I do not comprehend. As the words of these scrolls are consumed by my mysterious mind I will begin to awake, each morning, with a vitality I have never known before. My vigor will increase, my enthusiasm will rise, my desire to meet the world will overcome every fear I once knew at sunrise, and I will be happier than I ever believed it possible to be in this world of strife and sorrow." (Scroll I)

The scrolls each have a principle designed to replace bad habits built up over a lifetime which "threatens to imprison my future" (Scroll I) with good habits developed through a ten month process of studying the scrolls. Each Scroll's principle is life altering. They are:

• Scroll I - Today I begin a new life.
• Scroll II - I will greet this day with love in my heart.
• Scroll III - I will persist until I succeed.
• Scroll IV - I am nature's greatest miracle.
• Scroll V - I will live this day as if it is my last.
• Scroll VI - Today I will be master of my emotions.
• Scroll VII - I will laugh at the world (Keep perspective)
• Scroll VIII - Today I will multiply my value a hundredfold.
• Scroll IX - I will act now, I will act now, I will act now.
• Scroll X - I will pray for guidance.

Mandino's main philosophical message is that every person on earth is a miracle and should choose to direct their life with confidence and congruent to the laws that govern abundance. He wrote in Scroll I, "I will not fail as the others, for in my hands I now hold the charts (the Ten Scrolls) which will guide me through perilous waters to shores which only yesterday seemed but a dream." Og was also a proponent of taking action now. In Scroll IX, the phrase I will act now is written 18 times. He recognized that all successful people take on their own lives by "charting" or consciously choosing both the desired destination and the path to reach it. Mandino's philosophical messages have Christian undertones. They are encouragements to build good habits, to live according to the laws that govern abundance and to find a vigor and joy in life as we learn to stand at the helm of our ship charting its course into the safety of the harbor instead of just waiting for someone else to bring it in.

Mandino was married to his second wife Bette on December 9, 1957. He wrote a total of 19 books, 16 of which are still in print , the first being The Greatest Salesman in the World. He retired at the age of 52.

Mandino's hometown was Natick, Massachusetts. He was the oldest of three children.

The following books are what Mandino believed to be the twelve greatest self-help, self-knowledge, and self-inspirational books ever written:

• The Autobiography of Benjamin Franklin, by Benjamin Franklin
• Think and Grow Rich, by Napoleon Hill
• Success Through a Positive Mental Attitude, by W. Clement Stone and Napoleon Hill
• The Power of Faith, by Louis Binstock
• Your Greatest Power, by J. Martin Kohe
• Wake Up and Live, by Dorothea Brande
• I Dare You, by William Danforth
• Acres of Diamonds, by Russell H. Conwell
• The Ability to Love, by Dr. Allan Fromme
• How I Raised Myself From Failure to Success In Selling, by Frank Bettger
• The Magic Power of Emotional Appeal, by Roy Garn
• As a Man Thinketh, by James E. Allen
• The Holy Bible