Marguerite later found the book, Flip, by Wesley Dennis and loved the illustrations. She decided to send him one of her books, Justin Morgan Had a Horse, to see if he would illustrate it. He agreed, and the book won a prize as a Newberry Honor Book.
In 1945, Marguerite received a letter telling the tale of ponies that washed into the sea hundreds of years ago when a Spanish galleon crashed on a hidden reef. The ponies swam for the nearest shore at Assateague Island.
Marguerite decided to go to these islands on Pony Penning Day, when the ponies swim from Assateague Island to Chincoteague Island. Wesley Dennis came along. They found the story of a colt named Misty, and wrote and illustrated the book, Misty of Chincoteague. Later, Marguerite came back and bought the real Misty pony and took her on tours for children. Rand McNally published the book in 1947. It was a big success.
Next Marguerite wrote King of the Wind, which won the Newberry Medal. She wrote many other books, working with Wesley Dennis. For almost 20 years, she enjoyed his enthusiasm and good sense of humor. His drawings captured what she wrote in words.
Over the years, Marguerite and her husband Sidney had a pony, a burro, three fox cubs, a horse, two dogs, a cat and many kittens. She wrote 58 books in all. Children from all over the world wrote her letters, which she enjoyed very much.
Marguerite Henry died in 1997 at her home in Rancho Santa Fe, California.