If you look up William Byrd in an encyclopedia his birth date will often be succeeded by a question mark. It is, indeed, indefinite. It is just possible he was born as early as 1534, but more likely in 1543, depending upon whether the Wylliam Byrd who became a chorister in Westminster Abbey in 1543 is the composer, or simply another lost William Byrd. His birth place is usually given as Lincoln, since he has strong Lincoln associations later in life, but if the Westminster chorister and Byrd are the same, London is a more likely birth place. In any event, there is no doubt he died in 1623, aged at least 80.
It is a near certainty that Byrd sang in the Chapel Royal during the reign of Mary I under Thomas Tallis. In his mid-twenties he is found as organist and choirmaster of Lincoln Cathedral. He was named a gentleman of the Chapel Royal under Elizabeth, in 1572 and worked there as organist, singer and composer for many subsequent years. He published a collection of motets with Tallis before Tallis' death, and composed Ye Sacred Muses as an elegy to the departed Tallis.
In spite of Byrd's employment writing for the Protestant Church of England under Elizabeth, he seems to have harbored strong personal Catholic sympathies, and wrote a good deal of music for the Mass in his later years, apparently celebrating Mass secretly with his co-religionists. Even though Byrd composed and openly published Catholic music he was not molested by the state, though some of those in possession of his printed music certainly were. He composed prolifically throughout his very long life, and after Orlando Gibbons, is often considered the greatest of Elizabethan-Jacobean composers.