William McLean was born in Mason County, Kentucky and moved in 1799 with his parents Fergus and Sophia (Blackford) McLean and his older brother John McLean to a farm in Warren County, Ohio. There he attended the common schools, studied law and was admitted to the bar in 1814. He commenced practice in Cincinnati, Ohio and then was a lawyer at Lebanon, Ohio.

He removed from Lebanon to Piqua, Ohio about 1820 and was the first regular professional lawyer who settled in the village. He was receiver of public moneys and through his efforts a subsidy of 500,000 acres of land was procured for building the Ohio Canal from Cincinnati to Cleveland.

In 1822, William McLean was elected from Ohio's 3rd congressional district, which covered nearly all Western Ohio north of Warren County. He took his seat in the Eighteenth Congress. He was reelcted to the Nineteenth, and Twentieth Congresses. In the Twentieth Congress, he served as chairman of the House Committee on Indian Affairs.

William McLean returned to Cincinnati where he engaged in mercantile pursuits and the practice of law. He was also interested in agricultural pursuits. When his health began to fail, he retired from business and spent several months in Cuba hoping to derive benefit for his pulmonary disease by a the change of climate. His condition did not improve, and after returning to Cincinnati, he spent some time in revisiting several points in his old Congressional district.

William McLean died at his home in Cincinnati and was interred in the Catharine Street Burying Ground. In 1863, he was reinterred in Spring Grove Cemetery.