Walter Aitken was born in Streetsville, Ontario, Canada on June 17, 1869, the son of William and Janet MacGregor Aitken. He attended common schools and moved to New York City in 1886 where he learned the printer's trade. Aitken traveled to Montana in 1888 and sought work as a cowboy in the Shields River valley until resuming he resumed his newspaper career for the in 1893. He went on to establish newspapers of his own in Columbus, Big Timer, and Kalispell before becoming editor of the in 1904. All while working as a newspaperman, Aitken had been studying law and was admitted to the bar in 1906. He served as the city attorney for the towns of Belgrade and Bozeman, and stayed interested in civic affairs throughout his life. In February, 1909, Aitken represented a number of landowners on the West Gallatin River in an important Montana water rights case. Aitken married Florence N. Reese on May 22, 1898 and the couple had two children. Walter Aitken died on June 7, 1948.

In February, 1909, Aitken and fellow attorney John T. Smith represented a number of landowners on the West Gallatin River in their suit to establish their right to use water from the river. Since territorial days water rights in Montana had been based on the Doctrine of Prior Appropriation, which gave water use privileges to the earliest settlers on record. Problems consistently arose in establishing who the first settlers were, especially in local county records, and proving up a landowner's allocation to surface water was an expensive proposition. John M. Robinson, a Gallatin County farmer who had successfully introduced a bill in 1885 to establish Montana water law, became a plaintiff with dozens of other Gallatin County landowners who hired Aitken to press their claims in court. In this suit, 144 canals carrying water to 83,600 acres of land were involved, and one historian has estimated the legal fees may have been as high as 60,000 dollars.