Smith was born in Dowagiac, Michigan and attended the common schools. He moved with his parents to Grand Rapids in 1872, where he attended school, sold popcorn, and was a newsboy and messenger boy. He was appointed a page in the Michigan House of Representatives in 1875 (or 1879) at Lansing, Michigan. He studied law in the office of Burch & Montgomery (Marsden C. Burch was a one-time U.S. Attorney for the Western District of Michigan), studied law and was admitted to the bar in 1882. Mr. Smith practiced law alone for some time, but later became associated with Fredrick W. Stevens. This firm afterwards became Smiley, Smith & Stevens. He was general counsel of the Chicago and West Michigan Railway and the Detroit, Lansing & Northern Railroad Company. While in this practice, Mr. Smith became an expert on railroad law and finance. He was assistant secretary of the Michigan State Senate in 1883 and the State Game Warden 1887-1891, reportedly the first salaried state game warden in the nation. He was a member of the Republican State Central Committee from 1888 to 1892.

Smith was elected as a Republican to the Fifty-fourth Congress and to the six succeeding Congresses, serving from March 4, 1895, until his resignation, effective February 9, 1907, having been elected to the Senate. While in the House of Representatives, Smith was chairman of the U.S. House Committee on Expenditures in the Department of State in the Fifty-sixth Congress, the U.S. House Committee on Pacific Railroads in the Fifty-seventh and Fifty-eighth Congresses.

Smith was elected as a Republican to the United States Senate January 15, 1907, for the term beginning March 4, 1907. He was subsequently elected on February 6, 1907, to fill the vacancy in the term ending March 3, 1907, caused by the death of Russell A. Alger. He was reelected in 1913, and served from February 9, 1907, to March 3, 1919. He was not a candidate for renomination in 1918. Smith was chairman of the U.S. Senate Committee on Canadian Relations in the Sixty-first Congress, the U.S. Senate Committee on Territories in the Sixty-second Congress, and the U.S. Senate Committee to Examine Branches of the Civil Service in the Sixty-third through Sixty-fifth Congresses.

After the luxury liner Titanic sank in the North Atlantic on April 15, 1912, with more than 1,500 lives lost, Smith chaired Senate hearings that began at the Waldorf=Astoria Hotel in New York City the day after the survivors landed. Senators and spectators heard dramatic testimony from the surviving passengers and crew. Smith's subcommittee issued a report on May 28 that led to significant reforms in international maritime safety. Film director James Cameron used the Senate transcripts extensively in researching his 1997 Academy Award-winning film, Titanic.

Smith constructed a railroad in Michigan in 1898 and became owner of the Lowell and Hastings Railroad in 1900. In 1901 he was honored with the degree of Master of Arts by Dartmouth College. He was owner and publisher of the Grand Rapids Herald in 1906 and chairman of the board of directors of a transit company operating a line of steamboats from Chicago to various Lake Michigan ports. Smith died in Grand Rapids and is interred in Woodlawn Cemetery there.

The community of Alden, Michigan is named for him. Mr. Smith was married in 1885 to Nana Osterhout of Grand Rapids. They had one son, William Alden Smith, Jr.