William studied at the Brooklyn Polytechnic Institute and graduated from Princeton College in June, 1866 at the age of twenty, with the degree of A.B. Three years later he was awarded the degree of A.M. from his alma mater. While at college, William was prominent in athletic sports, and continued to take an interest in them later in life. He was a star catcher of the Nassau Club baseball team during his freshman to senior years.

William began his 50-year career at the firm of McKesson & Robbins at the bottom of the ladder in October, 1866. His strong work ethic and professional manner soon made him one of the firm's top salesmen. In 1870, he was made a partner and then put in charge of the manufacturing department, which experienced robust development under his management. For a time, he was one of only four partners, the others all being members of the McKesson or Robbins families (see partners). William was known for his strong emphasis on the quality and appearance of the firm's products. He also started the firm's Annual Prices Current, a huge publication used as a price list and reference book by druggists.

McKesson & Robbins was established in 1833 in Manhattan's financial district as a wholesale drug company that specialized in importing botanical remedies used to stock the medicine cabinets of sailing ships (see covered delivery wagon). By the time William entered the firm, the company had begun manufacturing its own drugs, including fluid extracts, tinctures, pills and tablets that won medals and worldwide recognition. During the early 1900s, the company became the nation's largest wholesaler of pharmaceutical drug products. It was later renamed McKesson Corporation, and by 2008 it had grown to be the largest health care company in the world, with annual revenue of $101 billion.

William was also secretary and treasurer of the Empire Coal and Coke Company, vice-president of the Mutual District Telegraph Company, member of the New York Chamber of Commerce, trustee of the Bowery Savings Bank, and of the executive committee of the Union League Club. He was also a member of the University, New York Athletic, Princeton, Racquet, Tennis, and Riding Clubs, the St. Nicholas Society, and the Metropolitan Museum of Art. He joined the American Pharmaceutical Association in 1870.

Although very busy with business affairs, William also had a passion for collectibles, and his house at 270 Park Avenue in New York was filled with many rare and lovely curios, collected from around the world. Antique weapons adorned his walls, while cabinets contained exquisite Japanese swords, netsuke ivory carvings, sword guards, silver pipes, and pipe cases, and other Japanese art forms. His collection of fine antique watches and snuff boxes was loaned out for exhibition at the Metropolitan Museum of Art for several years.

William married twice, first in 1871 to Electa Barrell Haines, with whom he had three daughters, Frances, Edith and Mabel. His second wife, Adrienne A. Pfaff, died in 1913, leaving an estate in excess of $1,000,000. William retired from McKesson & Robbins in December, 1916 and died at his home in New York City on November 21, 1925, with the funeral being held at St. Thomas Episcopal Church. The following year, his old employer McKesson & Robbins was purchased by Phillip Musica, a notorious swindler who used the company to orchestrate one of the greatest financial scams of the 20th century. The plot was eventually uncovered by company treasurer Julian Thompson, leading to the firm's recovery and Musica's suicide.