Both of William's parents were very wealthy due to inheritance. His mother inherited money from Shropshire and his father inherited money from the north of Ireland. According to legend, Betty’s father frivolously spent his money on anything, which resulted in his losing a large portion of his inheritance. This loss might have contributed to the extreme exploitation of his child prodigy, William.
William Betty first showed his desire for the stage at the age of eleven when, in 1802, his father took the young boy to Belfast to watch Sheridan’s Pizarro, starring Sarah Siddons in the role of Elvira. Her performance inspired him so much that William stated, "I shall certainly die if I may not be a player." Betty’s father introduced William to Michael Atkins, manager of the Belfast Theatre. After meeting the child, Atkins said, "I never dared to indulge in the hope of seeing another Garrick, but I have seen an Infant Garrick in Betty."
Not long after meeting Atkins, Betty was introduced to the theatrical prompter Thomas Hough, so he could direct, train, and mentor young William in the role of Osman in Voltaire’s Zair. While this was going on, there was an insurrection in Ireland which resulted in the closing of the Belfast theatre. Atkins knew he needed a huge attraction to bring in the crowds and he immediately thought of William. After some hard planning, it was settled, and on 11 August 1803, the eleven-year-old William Henry West Betty debuted professionally as the well-known Osman. His appearance brought in a large crowd, and reports stated that his performance was flawless and extremely well received.
He next took on the role of Young Norval in John Home's Douglas. This role fit him much better since he was actually playing a child and, once again, he astonished people in the theatre. News of Master Betty soon began spreading across Europe.
Master Betty’s fame extended beyond just Belfast to Dublin, where Betty’s father talked to Frederick Edward Jones, manager of the Crow Street Theatre. They were able to reach an agreement for Betty to appear again in Home's Douglas at the Theatre Royal, where he debuted on 28 November 1803. There he also played Frederick in Elizabeth Inchbald's Lovers' Vows, the title role in Tancred , and in William Shakespeare's Hamlet. It was said that in three hours of study he committed the part of Hamlet to memory. The citizens of Dublin became so excited over Betty that the civil authorities extended the curfew an hour for those attending the theatre.
His parents then had Betty tour in Scotland and England in 1804, where he was treated with thunderous applause as he reprised past roles such as Young Norval in Douglas. His performances sold out and earned nearly 850 pounds the last six nights. Home, the author of Douglas, came to watch Betty and claimed that he "considered it the only performance where Young Norval was played according to his conception of the character." Having become the biggest sensation in Dublin and Belfast, Master Betty was ready for London.
On 1 December 1804, guards were hired to handle the anxious crowd at the doors of the Covent Garden Theatre waiting to get a glimpse of the child sensation. Some waited in line for hours. Constables stood inside the theatre, ready to stem any chaos. Once the doors were open, people flooded inside to find seats, creating a huge disorder. Clark Russell described the event:
Shrieks and screams of choking, trampled people were terrible. Fights for places grew; Constables were beaten back, the boxes were invaded. The heat was so fearful that men all but lifeless were lifted and dragged through the boxes into the lobbies which had windows.
Master Betty played Selim in Brown’s Barbarossa or the Freedom of Algiers, an imitation of Voltaire’s Mérope. The boy did not come on stage until half-way through the show, but he was still grandly received by his audience, including the Prince of Wales. The second night, the patrons started a small riot, injuring many of the audience members and also damaging the theatre itself. At Drury Lane, the house was similarly packed, and he played for the then unprecedented salary of over 75 guineas a night.
Betty quit the stage in 1808 to attend Christ's College in Cambridge. After graduation, he lived with his family in the country, having become financially secure.
He was invited back to Covent Garden in 1812. The critics derided his performance, talking more about his former career as a child actor than his performance at the age of twenty-one. Betty never returned to perform in London again. Nine years later, he once again tried to mount a comeback and failed. He then tried to commit suicide, which also failed. He gave up acting in 1824.
Betty devoted the remainder of his life to works and theatrical charities. He died on 24 August 1874 in Ampthill Square, London.