Several published genealogies, without citing sources, claim William Kelsey was born 1600 in Chelmsford, Essex, England to George Kelsey Jr. and Elizabeth Hammond.  But as Anderson points out below, his origins are not known.
William KELSEY is not on any of the three passenger lists at the Mary and John Clearinghouse, and arrived in 1632 or 1633. It is a mistake to assign him as a passenger of the 1630 ship Mary & John.
The mistaken notion that William Kelsey's wife was a "Bethiah [?Hopkins]" can be traced to early reporting by a number of authors. In 1846, she was mis-identified by Hinman as "Bethia" the woman who married second to David Phillips.
Unless otherwise noted, the information that follows was abstracted from Robert Charles Anderson's 1995 work on the immigrant, William Kelsey.
William Kelsey was born about 1609, based on estimated date of marriage; origins unknown. [Gale Ion Harris says he was born "in England, about 1600, perhaps in county Essex"] 
He migrated in 1633, settling initially in Cambridge, Massachusetts, but removing first to Hartford, Connecticut in 1635 and last to Killingworth, Connecticut by 1668.
He married by 1634 _____ ____, who was "born about 1613 and living at Hartford in December 1666" [TAG 68:213-214]. (Several false leads regarding the identity of the wife of William Kelsey have been carefully examined and discarded by George E. McCracken and Gale Ion Harris [TAG 37:38-42, 68:211-14]. Based on onomastic evidence Harris suggests that her given name was Hester.)
One of the 25 Adventurers
Before the whole company of families traveled to their new town of Hartford, they chose twenty-five of the strongest men to go to that place in the wilderness, and to build shelters and break up the virgin soil and plant the early crops. They were to establish the company's claim to the land and defend against the possible intrusion of Indians or of the Dutch, who had a near-by settlement. William Kelsey is one of the twenty-five names inscribed for posterity on the plaque honoring the courage and fortitude of these first settlers of Hartford, Connecticut. See attached image.
Original proprietor of Hartford, Connecticut
"William Kelsey, born ab. 1600; Cambridge, 1632; freeman, Mass., March 4, 1635; removed to Hartford, where he was an original proprietor; his homelot in the distribution of 1639 was on the road from Centinel Hill to the North Meadow (now Village Street.); removed to Killingworth, ab. 1663; rep. in 1671 from K.; he was living in June, 1674."
On 21 Sept 1676, his will and inventory were presented at New London County Court; these documents do not, unfortunately survive. [TAG 69:28, citing New London County Court Records, Trials, 3:83]
Children of William Kelsey
William Kelsey and his wife had eight children, all but the eldest, Mark, were most likely born at Hartford. Mark was probably born at Cambridge.
Mark, b say 1634; m1 in Windsor 8 Mar 1658/9 Rebecca Hoskins [WiLR 1:58]; m2 in Windsor 26 Dec 1683 Abigail (____) Atwood, widow of Capt. Thomas Atwood [WiLR 1:58; CTVR 52 (giving only the year of the event)].
Hester, b abt 1636; m1 by 1656 James Eggleston, son of Bigod Eggleston; m2 in Windsor 29 Apr 1680 James Eno; m3 Windsor 10 Jun 1686 John Williams. (See TAG 68:208-10 for a detailed discussion of her marital career, which includes full documentation.)
John, b abt 1638; m by 1668 Hannah Desborough [TAG 38:210-11, 68:214]
Priscilla, b abt 1640 (aged 17 on 22 Dec 1657 [TAG 68:214, citing WMJ 74]); m by Jan 1659/60? Cornelius Gillett, son of Jonathan Gillett [TAG 68:214].
Mary, b abt 1644 (aged 14 in May 1658 [TAG 68:215, citing WMJ 109]); m in Windsor 23 Apr 1661 Jonathan Gillett [Grant 39], son of Jonathan Gillett.
Abigail, b Hartford 19 Apr 1645 [HaBOP 575]; m Killingworth 3 Dec 1668 John Hull [TAG 68:215]
Stephen, bp Hartford 7 Nov 1647 [HaBOP 578]; living 1670/1 [HaBOB 141-44] The Society of Descendants of of the Founders of Hartford adds: m. Nov. 15, 1672, Hannah, dau. of John Ingersoll; his father gave him his dwelling-house, and he remained in Hartford; d. Nov. 30, 1710.
Daniel, b Hartford July 1650 [HaBOP 581]; ;m1 Killingworth 27 Mar 1672 Mary Stevens [TAG 68:215]; m2 by 1693 Jane Chalker [TAG 68:231].
The Key to abbreviations found in the text quoted from Great Migration Begins: Immigrants to New England 1620 - 1633, Vol II, G -O, is as follows:
CTVR -- Births, Marriages and Deaths Returned from Hartford, Windsor and Fairfield and Entered in the Early Land Records of the Colony of Connecticut...., Edwin Stanley Welles, ed. (Hartford 1898)
HaBOP - Original Distribution of the Lands in Hartford Among the Settlers, 1639, Collections of hte Connecticut Historical Society, Volume 14 (Hartford 1912; rpt. Bowie, Maryland 1989)
TAG -The American Genealogist, published in print and online by the New England Historic and Genealogical Society [NEHGS]
WiLR - Windsor, Connecticut, Deeds (microfilm of original at Connecticut State Library, Hartford, Connecticut)
WMJ - Medical Journals of John Winthrop, Jr., 1657 - 1669, manuscript, Massachusetts Historical Society, Boston, Massachusetts
↑ Source: #S369Search for the Passengers of the "Mary & John 1630, Volume 1, Page: 32
↑ R. R. Hinman, ‘’A catalogue of the names of the first Puritan settlers of the colony of Connecticut; with the time of their arrival in the colony, and their standing in society, together with their place of residence, as far as can be discovered by the records. Collected from the state and town records’’ (1846), 44; digital images, ‘’Hathi Trust’’ (accessed 2014).
↑ Robert Charles Anderson, Great Migration Begins: Immigrants to New England 1620-1633, Vols I-III (1995), 3 vols., 2:1117-1119 "William Kelsey."
↑ Source: #S74185The Family of William^1 Kelsey of Hartford, Connecticut with notes on Hester^2 Kelsey, Wife of James^2 Eggleston, James^1 Eno, and John^1 Williams of Windsor, Connecticut, Gale Ion Harris, TAG Vol 68 (1993), page 210
↑The Colonial History of Hartford: Gathered from the Original Records, by William DeLoss Love, (1914), page 7 - 13
↑ Source: #S197 and #S401Great Migration Begins : Immigrants to New England 1620-1633; Vol. II, G-O, Anderson, Robert Charles, page 768 Text: ...daughter of William Kelsey. Updated version identified Hester
↑ Source: #S197 and #S401Great Migration Begins : Immigrants to New England 1620-1633; Vol. II, G-O, Anderson, Robert Charles, p. 768 Text: Priscilla....daughter of William Kelsey [TAG 68:214; Grant 39]
↑ Source: #S197 and #S401Great Migration Begins : Immigrants to New England 1620-1633; Vol. II, G-O, Anderson, Robert Charles, p. 768 Text: Mary Kelsey...daughter of William Kelsey [TAG68:209, 210, 215]
George E. McCracken, "The Wife of William Kelsey," The American Genealogist 37 (1960):38-42; digital images, AmericanAncestors.org (accessed 2014). [Note: This work is referred to and cited by Gale Ion Harris in "The Family of William^1 Kelsey of Hartford, Connecticut ..." The American Genealogist 68 (1993):208-215.]
The Great Migration Begins: Immigrants to New England 1620-1633, Volumes I-III. (Online database: AmericanAncestors.org, New England Historic Genealogical Society, 2010), (Originally Published as: New England Historic Genealogical Society. Robert Charles Anderson, The Great Migration Begins: Immigrants to New England 1620-1633, Volumes I-III, 3 vols., 1995). Pages 1117 - 1119
Source: S197Great Migration Begins : Immigrants to New England 1620-1633, Anderson, Robert Charles, Great Migration Study Project, New England Historic Genealogical Society ; Boston, 1995 Date: 09 Apr 2004, Vol. II, G-O Repository: #R175 Call Number: Gen Ref 974 An24g v.2 (G-O) Repository: R175 Saint Joseph County Public Library, 304 South Main Street, South Bend, IN 46601 USA, URL: www.libraryforlife.org
Source: S401Great Migration Begins : Immigrants to New England 1620-1633; Vol. II, G-O, Anderson, Robert Charles, Great Migration Study Project, New England Historic Genealogical Society ; Boston, 1995
The Colonial History of Hartford: Gathered from the Original Records, database, Google Books: 2008. Extracted from William DeLoss Love, Ph.D. The Colonial History of Hartford: Gathered from the Original Records. Hartford, Connecticut, Publisher: William DeLoss Love, 1914. pages 7-13.
Society of the Descendants of the Founders of Hartford.
Barbour, Lucius Barnes, 1982, Families of Early Hartford, Connecticut, Baltimore, Maryland: Genealogical Publishing Co. Inc. (1982), Baltimore, Maryland and Glastonbury, Connecticut: Connecticut Society of Genealogists, Inc. (1982), p.351
The Kelsey Genealogy Book 1, pages 24 to 38; Harleian Society Publications, Volume XIV, page 588; Bysshe's Visitation of Essex, page 55; Burke's General Armory Kelsey Family Website
Source: S276andS369Search for the Passengers of the "Mary & John 1630, Volume 1: Passengers & their children, Publication: Burton W. Spear, 5602 305th Street, Toledo, Ohio 43611, copyright 1985 - B. W. Spear, Fourth Printing - revised Date: 12 Aug 2006, Repository: #R209 Call Number: Gc 974 Sp3s v.1 Repository: R209The Genealogy Center at the Allen County Public Library, 900 Library Plaza, Fort Wayne, IN 46801-2270 USA, E-Mail Address: Genealogy@ACPL.Info
Find A Grave Memorial# 18761806 for William Kelsey -Burial at Indian River Cemetery, Clinton, Middlesex County, Connecticut. The information has not been updated to reflect the latest findings - use this information with caution. The death date and names of the mother and wife are particularly not viewed as correct due to later research and discovery.
William Kelsey was one of the original "Braintree Company" followers of the Reverend Thomas Hooker, who came to America and they were the first settlers of "New Towne" (now Cambridge) Massachusetts in 1632
It may be possible to confirm family relationships with William by comparing test results with other
carriers of his Y-chromosome or his mother's mitochondrial DNA.
Y-chromosome DNA test-takers in his direct paternal line on WikiTree:
I've suggested on her profile that she be detached not from William, but from her supposed parents, then merged into Hester Unknown, as there appears to be no evidence that her current supposed parents had a daughter named Bethiah.
This profile has some challenges. It includes a wife Bethia Hopkins not recognize by Anderson; it includes a daughter Bethia, also not included in Anderson, and a son William also not included. There may be other inconsistencies as well.