She was the Protestant daughter of the rake, playwright and
wealthy baronet, Sir Charles Sedley. As one of the Maids of Honour to
Mary Beatrice of Modena she caught the eye of her mistress's husband,
the Duke of York and future King James II. Like Mary Beatrice she was
young enough to be James's daughter, but unlike the Duchess she had
 little in her appearance to recommend her. She was skinny and, it was
said, she 'squinted', probably from being shortsighted.
Before she was sixteen, Pepys described her as 'none of the most
virtuous, but a wit'. She herself could not account for her success:
"It cannot be my beauty for he must see I have none, and it cannot be
my wit, for he has not enough to know I have any." However, she
retained James's attention but there is doubt whether the daughter
James acknowledged was in fact fathered by him. When this daughter,
Lady Catherine Darnley, grew up and took pride in her royal origins,
Catherine Sedley told her daughter: "You need not to be so proud for
Grahame having been Keeper of James's Privy Purse.
When James became King, Catherine Sedley was moved from her
Whitehall apartment and her pension doubled to œ4,000. The doubling of
her pension was meant to be compensation for not wanting to see her
any more, but before the year was over they were meeting again. A year
after his accession, James II made her Countess of Dorchester.
However, it was the longed-for son and heir which was the undoing of
James II. The English were willing to put up with one Catholic King
but not a Catholic dynasty and his Protestant son-in-law, William III,
was invited to replace him. James II, with his wife and son, fled to
France to spend the rest of his life as a pensioner of Louis XIV.
Catherine Sedley, now Countess of Dorchester, remained in England
where her father boasted about his support for the regime by
maintaining: "Well I am even with King James in point of civility, for
as he made my daughter a Countess, so I have helped to make his
daughter a Queen". Catherine had no qualms about attending the new
Queen's court, and, faced with Mary II's coldness on seeing her
father's ex-mistress, she had a typically brazen reply ready:
"Remember, Ma'am, if I broke one Commandment with your father, you
have broken another against him."
Her pension stopped and the grants of Irish land James II had
made her were called into question. However, she may have acted as a
double agent for William III who, afterwards, gave her a pension. In
her opinion: "Both the kings were civil to her, but both the queens
used her badly."
When nearly forty she found herself a husband, a one-eyed
Scottish baronet, Sir David Colyear, an officer in William III's army.
William III had a high regard for him and his military abilities and
later created him Earl of Portmore. In spite of her advancing years,
Catherine produced two sons. Perhaps the advice she gave her sons when
they went to school epitomises better than anything the coarse, earthy
quality that was her strongest characteristic. "If anybody call either
of you the son of a whore," she told them, "you must bear it, for you
are so; but if they call you bastards, fight till you die, for you are
an honest man's sons."

Source: Leo van de Pas