I have done considerable searching, including hiring a professional in Plymouth, to try to learn more about William's ancestry. Nothing!

There was one William, bapt. at Dedham on 22 Oct 1616, about whom no further record appears. He was the son of Richard Sherman, brother of Edmund (1572-1641) who married Joan Makin. Richard immigrated and died in Boston in 1660. I am suspicious that this may have been "our" William, and - either he had a falling out with his father - or they got separated somehow.

That is the only William I can find in any records to date who seems to be a possibility.

William Sherman: died in Marshfield, Plymouth Co., Ma. 25 Oct 1679; married 23 Jan 1638 at Plymouth, Ma. to Prudence HILL. It has been considered possible that William Sherman was identical with the man of that name mentioned in the records of the Massachusetts Bay Colony (vol. I, p. 25), under the date of 26 Feb 1628/9: "Wm Sherman hath liberty for 14 daies to fech his keynes in Northampt., near ferry." If this supposition is correct, he probably was about to emigrate then and was in the country before 1630, but there is no mention of him so early. He was in Plymouth before 1632. In 1640 he had lands granted to him at Green Harbor or Marshfield and removed there. He is first mentioned in the records of Marshfield, 25 Aug 1652. On 7 Jul 1653 he was made one of the committee with Josiah Winslow, Sr., Peregrine White, and Thomas Tilden to "agree with a workman or workmen" in regard to building a bridge over the South River. On 13 Aug 1657 he was chosen grand juryman for the year and, 11 Oct 1658, paid a tax of 10 shillings.

At at town meeting held 18 May 1663, the town granted to William Sherman and his heirs forever twenty acres of land to be added to the head of his estate. On 27 Apr 1668, he was appointed to sell the wheat 'now in the constables hands.'

He had apparently four adult children, three sons and a daughter. He evidently gave his grandson, William Stephens, some land, but the deed has not been found. He made no will and no administration was taken on his estate, but some years before his death, he gave to his three sons all of his real estate and, since he made no provisions for his wife in any way, it is presumed that she predeceased him. Unfortunately, the name of his daughter has not been preserved, but it was possibly Hannah or Sarah. The records of Marshfield have the following imperfect record: "[worn]h the wife of edward [worn] was Buried ye I of October 1680."1 It is supposed, and probably correct, to refer to the wife of Edward Stephens as this daughter of William's whose name cannot be found.

"To all people to whome this present writing shall come I William Sherman senr: of the Towne of Marshfield in the Collonie of New Plymouth in New England in America Planter sendeth Greet: &c [etc.]: Know yee that I the said William Sherman . . . for the tender Love and affection which I haue and beare to my Naturall and well beloved son John Sherman of Marshfield . . . have Given . . . all my right . . . unto one Share of a certaine prsell or Tract of land lying and being att and near Saconett in the collonie of New Plymouth . . . bounded with lands formerly Graunted to the Township of Plymouth on the Northerly syde and on the easterly syde: with the Lands of the Towne of Dartmouth; and with the sea on the southerly and westerly sydes . . . . unto my son John Sherman . . . In Witness whereof I have heerunto sett my hand and seale this fifth day of February; in the year of our Lord God 1673. . .

Signed . . . in the
prsence of us John Dogett
Samuel Sprague

Delivered in the psence
of Peregrine White
Francis Crocker

"William Sherman senir of the Towne of Marshfield in the Jurisdiction of New Plymouth . . . sendeth Greeting . . . Know yee that I the said William Sherman as well for and in consideration of the Great love and fatherly affection; which I beare unto my Naturall and wellbeloved son William Sherman of Marshfield . . . as alsoe for divers other good causes and consideration . . . have Given . . . and made over . . . unto my son William Sherman . . . all that my dwelling house and land . . . upland and meddow . . . within . . . Marshfield . . . together with all edifices buildings orchyards pastures . . . whatsoever . . . In witness wherof, I the said William Sherman senir have heerunto sett my hand and seale this fifteenth day of August Anno Dom 1676.

John Doged
Samuel Sprague
("This Deed was acknowlidged by William Sherman senir and hee declared the Intent of this deed was for all his Lands in Marshfield not mensioned in his son Samuells deed.")

Source: "The Descendants of William Sherman of Marshfield, Mass." by Mary Lovering Holman, pub. 1936

Thomas Townsend Sherman, in his book "Sherman Genealogy", pub. 1920 - pg. 13 - says, "William Sherman also came from England about 1632 to Plymouth, Mass., and afterwards settled at Marshfield where he died and was buried Oct. 25, 1679. His descendants are numerous. His place of origin in England is as yet unknown, but it was probably Northamptonshire." ." (no explanation for "why" he thought that)

From "The Great Migration Begins" authored by Robert Charles Anderson, NEHGS, Boston 1995 Beginning on page 1673 - William Sherman

Origin: Unknown
Migration: 1632
First Residence: Plymouth
Removes: Duxbury by 1638, Marshfield by 1644
Occupation: Yeoman
Freeman: In list of those in Duxbury who took the oath of fidelity in 1639 (PCR 8:182)
Education: Signed his deeds by mark.
Offices: Duxbury highway surveyor, 5 June 1644 (PCR 2:72). Marshfield highway surveyor, 7 June 1652 (PCR 3:9). Grand jury, 3 June 1657 (PCR 3:115).
In Duxbury section of 1643 Plymouth list of men able to bear arms (PCR 8:189). (Toward the end of the Marshfield section of this same list is an entry for "[blank] Sherman", who may be the same William who moved to Marshfield in 1643 or 1644 (PCR 8:196).
Estate: Assesed 9s. in the Plymouth tax list of 25 March 1633 (PCR 1:110); but absent from the list of 27 March 1634.
On 17 November 1637 James Davis of Plymouth, sailor, sold to William Sherman of Plymouth "all that his lot of lands lying near the lands granted to Mr. John Weeks containing by estimation five acres" (PCR 12:24).
On 5 February 1637/38 granted "a garden place on Ducksborrow side, & five acres of lands at Pouder Poynt, if it can be there had" (PCR 1:76). At court of 1 April 1639 William Sherman requested land (PCR 1:120). On 6 April 1640 William Sherman "is granted a meadstead about the Stony Brooke, in Duxborrow, and the said Will[ia]m & John Washborne to have such accomodations of land as may be spared in the place where they desire" (PCR 1:145). On 1 June 1640, along with six other men, granted four acres in Duxbury (PCR 1:153). On 2 November 1640 granted "twenty acres, his houselot to be part thereof,... northward from Duxborrow Mill, towards Greens Harbour" (PCR 1:165).
On 4 June 1648, "[d]ifferences depending between William Sherman and John Barker about the bounds of their lands, the Court do appoint and request Captain Myles Standish and Mr. Aldin, and to be accompanied with Joshua Prat, to set at right such differences as are between them" (PCR 2:127-28).
On 3 June 1662 William Sherman was one of the "servants and others that are ancient freeman" who were granted land by the General Court (PCR 4:18).
On 9 June 1673 William Sherman Senior of Marshfield, yeoman, deeded to "my natural son Samuell Sherman" of Marshfield a parcel of land and meadow "whereon I now dwell" together with "one half or moiety of the marsh and meadow land appertainingto my said lot" (PCLR 4:88). On 5 February 1673[/4?] William Sherman Senior of Marshfield, planter, deeded to "my natural and well beloved son John Sherman of Marshfield ... all my right ... unto one share of a certain parcel or tract of land lying and being at and near Saconett" (PCLR 4:408). On 15 August 1676 William Sherman Senior of Marshfield deeded to "my natural and well beloved son William Sherman of Marshfield ... all that my dwelling house and land ... upland and meadow" in Marshfield (PCLR 4:167).

Comments: William Sherman appears on the tax bill of 25 March 1633, but not in the list of 27 March 1634, and not in any other Plymouth Colony record for the next five years. This might be interpreted in a number of ways, including the possibilities that William Sherman returned to England, or that we are dealing with two different men of the same name. Were the gap much greater than five years, we would lean toward the latter solution, but in the absence of further evidence have concluded that only one William Sherman is represented by these records.
On 5 March 1638/9 William Sherman was presented "for drinking tobacco contrary to order" (PCR 1:118).
A William Sherman is mentioned in the records of the Massachusettes Bay Company on 26 February 1628/9 (MCBR 1:25), and some have thought that this might be the immigratn to Plymouth. It is more likely, however, that the William Sherman of the 1628/9 record was one of the many tradesman supplying goods for the ships bound for Salem, rather than being himself an immigrant.

1 There is a notation in Descendants of William Sherman of Marshfield, Massachusetts that states "if this date is correctly read she could not have been the wife of Edward Stephens, but it was probably the 7 of October, not the 1."