William married Mary Barstow on May 12, 1736 at Queen Anne's Chapel, an Episcopal Church in Newbury, Essex County, Massachusetts. 
Their children were all born in Hanover. In 1762 they moved to Harpswell, Maine. Their four youngest children moved with them and the older children remained in Massachusetts. William wrote in his diary:
May 25, 1762, then we left Hanover and came to Hingham that day shipped our good on board the Grayhound, Courtney Babbidge, Commander, and we sailed that night about 10 o'clock. The 26 day we went on board. 30 we came to anchor and landed our cattle. 31 we landed our goods and took possession of our house in Harpswell the last day of May.
During the American Revolution William was a member of the committee of supply and safety. Harpswell, Cumberland. 
Maine remained a province of the state of Massachusetts until 1820 when it was admitted to the union as the 23rd state. Harpswell was incorporated by the Massachusetts General Court in 1758. Before that the area was a part of North Yarmouth.
In Harpswell "he was an an industrious and prosperous farmer and was prominent in the affairs of the town."
William was listed in the events and decision-making of the town in the time leading up to and during the Revolutionary War.
At the January 28, 1773 meeting of the Harpswell district a "communication from the town of Boston, setting forth the rights of the colonies and a statement of the infringement of their rights, was laid before the town for their consideration. A committee, consisting of William Sylvester, Deacon Andrew Dunning, and Captain Benjamin Jaques, was chosen to write a reply and to submit it to the town at a special meeting to be held on the eighth day of March, at which time the committee reported" At that point the report was quite hopeful to remain loyal to the British, "we Concur with the Sentiments of the Committy of Correspondence at Boston as they have stated the rights of the Colonists and of this Province in Particular, and of the Infringements on those Rights. We openly and Frankly declare that we hold Fast our Loyalty to our Sovereign (Independency we have not in View -we abhor the Thought) and hold our Selves in Readiness at all times with our lives and For-tunes to assist his Majesty in his Defense as we have heretofore done (when for the Enlargement of the British Empire, upon the Ruins of their Perfidious French Neighbors we have Cheerfully Emptied our Purses and furnished out our Quota of Men to join the Countless Numbers of loyal Americans who have Sacrificed their lives in the high Places of the Field & Desert -Many of whose Bones are to this Day whitening in the sun) yet we groan under our Burdens we sensibly feel them,"..." May we hope for Redress by Frequent application to our Gracious and good King which application we humbly conceive." 
A meeting was held on August 11, 1774  William was part of a committee to reply to letters from Boston concerning entering into a agreement for the non-consumption of British goods. The reply included; "it is our opinion that our now coming into the non-consumption agreement previous to the result of the Congress would be premature and that we should anticipate the very end of that respectable body we therefore think it more regular and advisable first to hear the measures advised to by the Congress and we hold ourselves in readiness to comply with the same." The report was accepted.
By March 20, 1775 the attitudes toward Independence had changed and the town passed a vote to " give the men that shall engage as Minute Men too (sic) shillings & eight pence per week allowing they meet three days each week & spend three hours each day in exercising or learning the art military for one month from date agreeable to the advice of the Congress" William Sylvester was among others chosen to be in a Committee of Inspection and of Correspondence. 
On January 16, 1776 at a town meeting a committee was chosen to carry a petition to the General Court " the necessity there was for an armed guard and a supply of ammunition in the town." On March 4, William was one of the members chosen for a Committee of Correspondence, Inspection, and Safety. On May 6 they recinded the pay for Minute Men. On July 30 "it was voted that the selectmen should take charge of the ammunition and arms, receipt for the same, and then distribute them among the officers of the militia, who should be accountable for them."
William did not appear in records after1776 that I have located. He would have been in his sixties at the time.
The family appeared in the 1790 U. S. Census living between "Sandy River from mouth to Cass Plantation" in Harpswell, Cumberland County 
He died on June 17, 1799 at the age of 91. He was buried at Old Harpswell Common Burying Ground in Harpswell Center, Cumberland County, Maine.  His obituary was published in the Columbian Centinel on July 13, 1799.
The surname was often spelled "Silvester" in most records during this time period.
List from Genealogies of Mayflower Families, from the New England Historical and Genealogical Register 
Buried Harpswell Center, Cumberland County, Maine, United States of America. 
Marriage Husband @P1517@. Wife @P1516@. Child: @P1399@. Marriage 12 May 1736. Newbury, Essex, Massachusetts. 
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