Queen Elizabeth ascended to the throne in 1952. She has now ruled for 59 years and 95 days.
Queen Victoria is the only monarch to spend more time on the throne. The 85-year-old queen would surpass Victoria's length of service in September 2015.
The longevity of Elizabeth's reign pushes George III into third place.
Last month, Prince Charles broke the record for spending the longest time as heir-to-the-throne.
The Associated Press contributed to this story.