This article written and copyright 2018 by Will Johnson, wjhonson@aol.com, Professional Genealogist
William Cecil was born in 1590 slightly before a letter, dated 8 Jun 1590, from his great-grandfather Lord Burghley written to John Manners to thank him for being the child's godfather at his christening at Newark (which is in co Nottingham) . William Cecil was the only child of Elizabeth Manners, Baroness Ros suo jure, by her husband William Cecil later the 2nd Earl of Exeter (from 1623). I identify this John Manners, whom Burghley only calls "cousin" of the child, as that John latter knighted, who was the second son of Thomas Manners, 1st Earl of Rutland. He would then be the second-cousin once-removed of the infant William Cecil.
Elizabeth had been the only child of her father Edward Manners, 3rd Earl of Rutland and Baron Ros, by his wife Isabel Holcroft. When Edward died 14 Apr 1587, the title Earl of Rutland went to his brother as his heir-male, but the Barony of Ros descended to Elizabeth aged "11 years and 4 months" at his death. She married William Cecil in January of 1587/8. At that time, the calendar year did not tick over until the end of March, so they called it 1588, but we today would have called it 1589. That same year he was elected a Member of Parliament, for Stamford.
Elizabeth died 11 Apr 1591 and was buried at All Hallows, Barking. Her son William did not immediately become Lord Ros, but rather there was a battle over this title, with the heir-male of Elizabeth (her first-cousin Francis Manners 6th Earl of Rutland), and Cecil was only finally granted that title in 1616.
Sometime between his first wife's death in 1591, and 1594, William Cecil (the elder) married Elizabeth Drury, the daughter of Sir William Drury of Hawstead, co Suffolk, who had been Lord Deputy of Ireland, by his wife Elizabeth Stafford. Her younger sister Diana Drury, later married William's younger brother Edward Cecil.
On 13 Feb 1615/6, William Cecil (the younger) married Anne Lake, the eldest daughter of then Secretary of State, Thomas Lake of Canons, by his wife Mary Ryther, daughter of a past Lord Mayor of London, William Ryther. Several secondary sources call this girl "Elizabeth" but this is a mistake for her sister Elizabeth Lake who married in 1622 to William Stonor of Stonor, esq, and later knight.
Sources and Further Reading:
ODNB, 'Cecil, William, 16th Baron Ros (1590-1618)' by Alastair Bellany (2004); as cited by Leo van de Pas, Genealogics (below)
"William Cecil, 2nd Earl of Exeter", Genealogics
"William Cecil, 2nd Earl of Exeter", Wikipedia
"William Cecil, 16th Lord de Ros of Helmsley", thepeerage.com