William was born to Daniel Schlich and Charlotte Frank. Both parents came from Hessian families and Daniel was a Lutheran pastor or Kirchenrat. His early education was at Flonheim and then at Langgöns and other schools in Hesse where the family moved. Schlich attended the Gymnasium in Darmstadt (1851).
In 1855, he entered the University of Giessen, where he studied under Gustav Heyer (1826-1883). Graduating in 1862, he joined the Hesse forestry service and was appointed Oberforster in Homberg in 1865. He received a doctoral degree in 1867 from Giessen. The Austro-Prussian War of 1866 forced him to move, and, on Heyer's recommendation he entered the British Imperial Indian Forest Service. Arriving in India in February 1867 his first posting was in Burma. He was promoted and worked in Sind and later Bengal, becoming Conservator of Forests in 1871, and Inspector-General of Forests in 1883, succeeding his mentor Dietrich Brandis. He developed forest management and education programmes and spent 19 years in India, helping to establish the journal Indian Forester in 1874 (becoming its first honorary editor) and the school at Dehradun in 1877.
In 1885 Schlich moved to England to take up the pioneering post of Professor of Forestry at the Royal Indian Engineering College at Cooper's Hill, near Egham, Surrey, the first formal forestry course in England. He became a British citizen in 1886. In 1905, upon the closure of the college at Cooper's Hill, he moved to Oxford, to found Oxford's forestry programme. He retired on 1 January 1920 and lived on at Oxford where he died on 28 September 1925 from a bronchial infection. He is buried at Wolvercote.
Schlich was a colleague and mentor of Gifford Pinchot. He was made a Fellow of the Royal Society in 1901, awarded the Knight Commander of the Indian Empire in 1909 and was an Honorary Fellow of St John's College.
Schlich was the author of the five-volume Manual of Forestry (1889–96) published serially in three editions. The first two volumes were on silviculture, the others dealing with forest management, forest protection, and forest utilisation. His Manual became the standard and enduring textbook for forestry students. In 1904 he published Forestry in the United Kingdom. Other publications were The Outlook of the World's Timber Supply and Afforestation in Great Britain and Ireland.