Born in Milford Haven, Pembroke County, Wales - the son of Capt. Stephen Raines and Anne Kingdom. In 1803, he entered Royal Military College at the age of 14; in 1803, he was commissioned a Second Lieutenant at the age of 16; from 1807 - 1813 fought in the Peninsula Wars during the Napoleonic Wars. Married Ann Eve Archer Williams in 1811 in Italy. Fathered 6 children with her. His marriage to Ann Eve ended at about the time of the birth of their 6th child in 1821. He met and had a child with Frances Doubleday (who would become his common law wife) in 1829. In June of 1830, William and Frances left along with two of his sons from his first marriage, his son with Frances and with Frances's sister, Eliza/Elizabeth. They came to Canada. They settled in Sutton Bay on Lake Simcoe. In 1835, they set sail from there to settle on St. Joseph Island. He created a community called Milford Haven on the east side of St. Joseph Island. At some point, he became disillusioned/disappointed and relocated to a place near the Fort called Rains Port. He and his family also spent time on the US side of the island now known as Neebish Island, Michigan. It was Maj. William Rains that gave Sailors Encampment its name because of the many lake bound ships who would become ice bound each fall and tie up there for the winter.
Not only did William have 13 children with his common law wife, Frances, but also had 6 children with her sister, Elizabeth/Eliza. Each "wife" had a home, and although their situation was considered "unconventional", this arrangement was said to have worked out well overall.
Rains was an accomplished linguist and scholar training each of his children in history, literature and politics. He is said to have had the largest private library north and west of Toronto.
While Rains was an individual success, he was ahead of his time and the settlement he envisioned on the island did not develop until he was an old man. His descendants would establish on the island the first frame house and the first two-story house. In 1875/6, they also helped to build the Island’s first schoolhouse. The Island’s first Church, aptly named "The St. Joseph Chapel" was built on the land owned by two of the Major's sons. It opened its doors in 1876 as a Roman Catholic church and is now known as the nondenominational "Church of Mary" located in Sailors Encampment.
Major William Kingdom Rains died on the island on October 19, 1874. He was buried in Greenwood Cemetery in Sault Ste. Marie with military honors.