William Mulholland was born on September 11, 1855, in Belfast, Ireland. His father Hugh Mulholland and his mother Ellen Deakers Mulholland were originally from Dublin and they moved back a few years after William's birth. Ellen Mulholland died when William was seven years of age and William grew up in a world dominated by men. He attended the O'Connell School run by the Christian Brothers. Hugh Mulholland had been born in 1827 during the first of the famines that plagued Ireland for forty years. At the time of William's birth he worked as a guard for the Royal Mail, a position which moved him into the emerging lower middle class. Even so life was hard and William remembered his childhood as being unpleasant and difficult. (Oscar Wilde and George Bernard Shaw also grew up in Dublin during this period.) In 1865 Hugh married Jane Smith, but William's foster mother appears to have had little impact on him. After being beaten by his father for receiving bad marks in school, William ran off to sea. By the age of fifteen he was a member of the British Merchant Marine. He spent the next four years as a seaman sailing primarily in Atlantic waters.
On June 9, 1874, William sailed into New York Harbor and decided to stay in the United States. That summer he worked as a deck hand on the Great Lakes. The following winter he worked in a lumber camp in Manitee, Michigan. After sustaining an injury to his leg he gave up life as a lumberjack and traveled widely as an assistant to an itinerant mechanic. In the fall of 1875 after meeting up with his brother, Hugh Patrick, he sought out his uncle in Pennsylvania. Richard Deakers, a brother of William's mother lived in Pittsburgh and operated a prosperous dry goods store. The two brothers were accepted into the Deakers family and worked in the store for two years. Late in 1876 the family decided to move to southern California in order to avoid a wave of tuberculosis that was then running rampant in Pittsburgh. They travelled via Panama where Hugh Patrick and William were forced to hike the forty seven miles across the Isthmus due to a lack of money for the train fare. Once on the west coast of Panama they shipped out on a Peruvian naval vessel to Acapulco. In Acapulco they managed to find births on another ship bound for San Francisco. In San Francisco they purchased horses and turned south in search of the Deakers family.
The two Mulholland brothers arrived in Los Angeles in January 1877. The town with a population of about 9,000 was not much to look at and William quickly decided to return to life at sea. On his way to San Pedro to find a ship, he was offered and accepted a job digging a well. After a brief stint in Arizona where he prospected for gold and worked on the Colorado River, he obtained a job as Deputy Zanjero with the newly formed Los Angeles Water Company (LAWC). (In California during the Spanish and Mexican administrations water was delivered to Los Angeles in a large open ditch, or zanja. The man who tended the ditch was known as a zanjero.) In 1880 Mulholland worked under LAWC Supervisor Frederick Eaton overseeing a crew laying the first iron pipeline in Los Angeles. Mulholland left the employment of the LAWC briefly in 1884 but returned in mid-December of that same year. he left again in 1885 and worked for the Sespe Land and Water Company. As part of his compensation he was granted twenty acres on Sespe Creek. In 1886 he returned to the LAWC and, in October of that year, became a naturalized American citizen. At the end of that year he was named superintendent of the LAWC.
Los Angeles' first land boom commenced in 1886 and the population increased dramatically from about 10,000 to 50,000 people in about three years. As more and more people were attracted to Southern California the availability and distribution of water became increasingly important. On July 3, 1890, William married Lillie Ferguson and proceeded to raise a family. During the 1890s land developers in Southern California continued to build new communities and expand old ones. One of the principal figures during this period was Frederick Eaton. Eaton was elected Mayor of Los Angeles in November 1889 and served through 1900. The mayor favored the formation of a Municipal Water Company to replace the LAWC and had first visited the Owens Valley in 1880 where he had conceived of the idea of bringing Owens Valley water to Los Angeles. The city's population now stood at 100,000 persons and he firmly believed that its future water requirements could not be met with existing water sources in Southern California.
In 1902 the City of Los Angeles succeeded in purchasing the LAWC and William Mulholland was retained as Superintendent. During the early fall of 1904 Mulholland and Eaton quietly traveled to the Owens Valley where Mulholland was convinced that Los Angeles should move immediately to obtain water rights in the valley. Several other entities including local residents, land and water companies, power companies, and the federal government were also considering what should be done in the valley. The battle to control the run off from the eastern slope of the Sierra Mountains proved to be acrimonious in the extreme and periodically erupted in violence, but eventually the City of Los Angeles succeeded in purchasing water rights throughout the valley. Work on an aqueduct started in July 1907 and was completed in June 1913 at a cost of $24.6 million. The first water flowed into San Fernando on November 5, 1913. A trunk line delivered water to Los Angeles proper in June 1914. Mulholland was credited by friend and foe alike as having been the driving force in the entire endeavor.
By 1914 Los Angeles was still expanding and already had a population of over 500,000 persons. Even with the flow from the Owens Valley aqueduct it was in need of more water. Los Angeles reached higher up in the valley to Mono Lake and added more watershed to its holdings. (By 1920 the city's population was to exceed 730,000 persons.) As growth continued in southern California, the city's attention turned to the Colorado River and Mulholland quickly became involved in the struggle to harness that river for power and water. (Eventually, after Mulholland's death, Boulder Dam and the Colorado River Aqueduct would be built.) At the end of 1924 Mulholland Highway was officially dedicated in appreciation of his services to the city. He was feted by his supporters as being an engineering genius for building the more than two hundred miles of aqueduct through mountains and desert and was attacked by his detractors for robbing Owens Valley of its water.
On March 11, 1928, Mulholland inspected leaks in the Saint Francis Dam in San Francisquito Canyon that had been built under his supervision during 1924-1926. His inspection found no fundamental flaws in the dam. Late that night, just before midnight, the dam failed and killed over 400 people. Even though the ultimate cause of the failure was never confirmed to his satisfaction, a devastated Mulholland accepted the blame for it saying only that he must have overlooked something. He was replaced as head of the water company in March 1929. He died in his home on July 22, 1935.