The leading rider achieved the landmark aboard the Dean Ivory-trained Stake Acclaim in the Shergar Cup Dash, a victory which helped the Great Britain and Ireland team to win the overall competition in Berkshire.
Only Frankie Dettori, Ryan Moore and Joe Fanning of the current crop of jockeys have reached the landmark.
Spencer won the Oaks in 2009 on Michael Bell's Sariska, the 2011 Gold Cup at Ascot on Fame And Glory and the 2003 St Leger on Brian Boru among many top level successes.
"I wasn't aware it was approaching until someone mentioned it on the way to Ascot on Saturday," Spencer said.
"You won't really look back on your career until you stop riding, so I'll just keep looking forwards until then.
"There have been some good days. I've been very fortunate to have done well at Royal Ascot over the years and winning the Oaks.
"Some of the smaller ones are sweet, too. When you win for an owner that hasn't had a winner before, sometimes that can be as satisfying."