The post was written while the monarch was visiting the Science Museum in London. While there, she saw a letter written to one of her ancestors, which is featured in the photo she posted.
"Today, as I visit the Science Museum I was interested to discover a letter from the Royal Archives, written in 1843 to my great-great-grandfather Prince Albert," she wrote.
PHOTOS: Queen Elizabeth through the years
She went on to explain that the letter was from Charles Babbage, an inventor who was writing to Prince Albert about his "Analytical Engine," which Ada Lovelace then used to create the first computer programs.
The queen reflected that there was no better place to make her first Instagram post.
"Today, I had the pleasure of learning about children's computer coding initiatives and it seems fitting to me that I publish this Instagram post, at the Science Museum which has long championed technology, innovation and inspired the next generation of inventors."
This is not her first venture into social media. In 2014, the queen tweeted for the first time.
In other royal family news, Duchess Meghan made a surprise appearance to join Prince Harry at WE Day UK in Wembley Stadium earlier this week. The crowd of young people greeted the American actress-turned-royal with cheers.
The duchess is expecting the couple's first child, Queen Elizabeth II's eighth great-grandchild, sometime in April.