In fact, he actually excelled at mathematics throughout his schooling and even considered becoming a mathematician for a time.
This rumor actually started while he was still alive and even showed up in a particular issue of Ripley’s Believe It Or Not. Einstein was even shown this article in Ripley’s, which had the title “Greatest living mathematician failed in mathematics.” Beyond the fact that this failure never happened, the irony of this was also that Einstein was not even a mathematician. Einstein reportedly found the article humorous and remarked: ““I never failed in mathematics… Before I was fifteen I had mastered differential and integral calculus.” (15 was the age in Germany that most qualified students would start to learn calculus.)(more)