William Byrd was born in 1539/1540 in London, England to Margery and Thomas Byrd. There were seven children in the family. William's two older brothers, Symond and John, sang in the choir at St. Paul's Cathedral. It's therefore likely that William received some sort of musical training from a young age.

Byrd had the good fortune to become a pupil of Thomas Tallis, the most prominent and talented composer in 16th century London. Byrd studied organ and composition with Tallis. He and Tallis would become lifelong friends and business associates.

In 1563, Byrd was appointed organist at Lincoln Cathedral. He met and married Julian Birley there. They had several children, including one named Thomas, after Thomas Tallis.

While serving at Lincoln Cathedral, Byrd composed numerous pieces for choir, violin, and keyboard. These works show an Italian influence and are written in a polyphonic style.

Byrd returned to London in 1572 and was appointed organist at the Chapel Royal, a position he shared with Thomas Tallis. Together, they provided the worship music for the royal household. Queen Elizabeth, perhaps in a show of gratitude, granted the two composers a monopoly on the printing of music. They immediately published a collection of sacred songs dedicated to the Queen.

Byrd was a prodigious composer. He wrote music for every genre of his day. There are volumes of sacred songs suitable for both Catholic and Anglican worship. There are secular songs in English. There are pieces for choir, pieces for soloists accompanied by viols, pieces for viol ensembles, and collections of compositions for keyboard. In all of these genres, he did outstanding work.

According to study.com