Queen Elizabeth II became the longest-reigning British monarch on 9 September 2015 when she surpassed the reign of her great-great-grandmother Victoria. On 6 February 2017 she became the first British monarch to celebrate a sapphire jubilee, commemorating 65 years on the throne.

Who has been the longest reigning british monarch? OK. Let's think. My stream of consciousness:
To answer this question, we should know that: Queen Elizabeth II became the longest-reigning British monarch on 9 September 2015 when she surpassed the reign of her great-great-grandmother Victoria. The answer is Queen Elizabeth II.