William Allan Neilson was born March 28, 1869, in Doune, Pertshire Scotland. He attended Edinburgh University where he studied philosophy, graduating with honors in 1891. In order to join his brother who emigrated to Canada, Neilson crossed the Atlantic and began teaching in Upper Canada College in Toronto which was a boarding school for young boys.

Deciding that he would prefer to be a scholar than a school master, Neilson went on to Harvard Graduate School in Cambridge, MA where he received his Ph.D. in English in 1898. Upon receiving his Ph.D., Neilson accepted a position as an Associate in English Literature at Bryn Mawr College in Bryn Mawr, PA. Neilson taught at many other fine institutions before settling into the work of an administrator. Harvard, Radcliffe, the Sorbonne, Columbia, Barnard, and the University of California all made use of Neilson's talents at one time or another.

It was during his years as a professor that Neilson distinguished himself as the sort of scholar that an academic institution would wish to guide it. Before coming to Smith, Neilson made a name for himself both as a professor devoted to his students' learning (he was the only instructor of Helen Keller's who learned the manual sign language she used for communication) but also as a writer and an editor. Among his writings were Essentials of Poetry (1912)and The Facts about Shakespeare (1913). He also was the editor of Milton's Minor Poems (1899) and Shakespeare's Complete Works (1906).

In 1917, Neilson was offered the Presidency of Smith College. He accepted on the conditions that he would not be obligated to raise money, and that a president's house would be built, with which the trustees complied.

Perhaps one of the best assets that Neilson brought to Smith was his quick wit that endeared him to student, staff, faculty, and alumnae alike. At Neilson's final commencement in 1939 Cynthia Walsh recalled one particular instance:

The seniors will especially recall that on May Day when they went to serenade him, his new collie puppy began to bark, not realizing what sweet voices were vying with the birds so early in the morning. When the president stepped out he said, "My dog thinks you're sheep," and the volleys of laughter which greeted this remark quite drowned out the rest: "-but I think you're lambs!"

Neilson's accomplishments at Smith were many. When Neilson noticed that a majority of Smith students lived off campus he set about the task of purchasing and building new residences so that all Smith students would live on campus. After building the Great Quadrangle, purchasing Chapin House, Talbot House, Sessions House, and many others his goal was completed. During Neilson's 22-year term as president, the campus also saw the additions of Sage Music Hall, Scott Gymnasium, Tryon Art Gallery, the Alumnae House, the Infirmary, and the President's house.

Neilson also established the Junior Year in France, Junior Year in Italy, and the Junior Year in Spain Programs. Special Honors, the Smith College Day School, and the School for Social Work were additions that Neilson made to the college.

Never one to rest on his laurels, Neilson continued writing and editing while looking after the well being of 2,000 students. In 1920 he wrote History of English Literature and in 1922 he edited Roads to Knowledge. He also was the editor-in-chief of Webster's New International Dictionary, 2nd edition, 1934.