William Greenough Thayer (December 24, 1863 – November 27, 1934) was an American educator, headmaster of St. Mark's School from 1894 to 1930.

This article includes a list of references, related reading, or external links, but its sources remain unclear because it lacks inline citations. (June 2018)

Thayer was born in New Brighton, Richmond County, New York, the son of Robert Helyer Thayer, a hardware merchant and ship chandler, and Hannah Appleton.

After graduating from Amherst College in 1885, he attended the Union Theological Seminary and Episcopal Theological School (Cambridge), he was ordained an Episcopal minister.

Thayer taught at Groton School, from 1889–1894, before being appointed headmaster of St. Mark's School, a post he held for 36 years until 1930. During this period, Thayer made numerous improvements to the school and its reputation rose to be amongst the most prominent private schools in the country.

Eight or ten times in the school year, Thayer would leave school to marry his alumni. From the Time article: Imposing is his Record at Socialite Weddings, for Loyal St. Mark's Grooms will have no other Cleric.

He was known by staff and students as "Twill".

His wife was Violet Otis (January 17, 1871 – November 16, 1962), daughter of William C. and Margaret (Sigourney) Otis of Boston.

They married in Boston on June 1, 1891. They had five sons and two daughters:

1. Sigourney Thayer 1896-1944
2. Robert H. Thayer 1901-1984
3. John Otis Thayer (died 1988)

• "Twill". Time Magazine. December 2, 1929. Archived from the original on July 15, 2010. Retrieved August 9, 2008. Time article on his retirement in 1930
• The Rev. William Greenough Thayer | Rectors and Vicars | Parish History | St. Andrew's Episcopal Church - Ayer, Massachusetts at www.standrewsayer.org Biography