Margaret was born in 1541, the youngest child and only daughter of Thomas Fiennes, 9th Baron Dacre and Mary Neville.

In the year of her birth, her father was hanged for the murder of a gamekeeper by the order of King Henry VIII, and his lands and title were forfeited to the crown.

On 10 November 1564 at the age of 23, Margaret married Sampson Lennard (died 1615), who came from a family of landed gentry.

Lady Dacre and her husband had five sons and six daughters:[1] Occurring in Sevenoaks, Kent, England unless otherwise stated: The title of Baron Dacre had been restored to Margaret's brother Gregory by Queen Elizabeth I shortly after her ascension to the throne; however, upon his death in 1594, it had once again lapsed in abeyance.

On 8 December 1604, King James I created her suo jure Baroness Dacre, and she held this title until her death on 16 March 1612.