WILLIAM GREENOUGH THAYER, son of Robert H. and Hannah F. (Appleton) Thayer, was born 24 December 1863 in New Brighton, Staten Island, New York. His father was a hardware merchant and ship chandler. He was fitted for college at Orange Classical Institute in Orange, New Jersey. He was graduated from Amherst College in 1885 with a B.A. degree. He commenced his studies for the ministry at Union Theological Seminary in New York from 1885-1886, taught at Groton School in Groton, Massachusetts from 1886-1887, then returned to Union Theological Seminary from 1887-1888. He was awarded an M.A. degree from Amherst College in 1888. He attended Episcopal Theological School in Cambridge, Massachusetts from 1888-1889 and received a B.D. (Bachelor of Divinity) degree. He was ordained a deacon in June 1889 by the Rt. Rev. Henry Codman Potter, Bishop of the Diocese of New York, and a priest in May 1890 by the Rt. Rev. Benjamin Henry Paddock, Bishop of the Diocese of Massachusetts.
He returned to Groton School as an English teacher in the fall of 1889. The Rev. Dr. Endicott Peabody, headmaster at Groton School, also assigned him to begin the work of founding an Episcopal mission in the nearby town of Ayer, Massachusetts. The first service was held in the lower town hall on Sunday evening, 20 October 1889. The evening services continued in the town hall for a little more than three years. On 29 August 1892, ground was broken for a new church building, built of grey granite with a shingle roof, to be known as St. Andrew's Episcopal Church. Fifty years later, Mrs. Thayer wrote, "How well I remember our joy and excitement over every stone that went into it." The Rev. William G. Thayer was soon after appointed Minister and on Christmas Day 1892, the first service was held in the new church. He was married to Violet Otis, daughter of William C. and Margaret (Sigourney) Otis of Boston, 1 June 1891 in Boston. They had seven children.
It would be impossible to relate all that Rev. Thayer accomplished during his five years at St. Andrew's, as a result of his hard work and untiring devotion. In July 1894, he was elected headmaster of St. Mark's School, an Episcopal secondary school in Southborough, Massachusetts. On 8 August 1894, it was with the deepest regret that the parish listened to his last sermon, realizing that they were parting with a real friend and faithful leader. Happy and valuable memories remained with those who were privileged to associate with him. St. Andrew's Episcopal Church is a living memorial to his Christian service.
After leaving Groton School and St. Andrew's Church, Rev. Thayer became both headmaster of St. Mark's School and also rector of St. Mark's Episcopal Church, in Southborough, positions he held until his retirement in September 1930. He became one of the principal figures in private education in the United States. In
The Education of the Modern Boy, which he authored together with the headmasters of five other New England private secondary schools in 1925, he provided an insight into his philosophy while headmaster at St. Mark's School: