William Greenough Thayer (December 24, 1863 – November 27, 1934) was an American educator, headmaster of St. Mark's School from 1894 to 1930.
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.
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.
From the Time article: Imposing is his Record at Socialite Weddings, for Loyal St. Mark's Grooms will have no other Cleric.