Users will be able to follow the Secret Service via @mi5official where they can take part in Q&A sessions with serving intelligence officers and see career opportunities.
The intelligence agency also plans to use the platform to bust popular myths and reveal never-before-seen material from its archives.
The account's first post appeared on Thursday morning and urged people to "consider all angles".
The post read: "The secret to successful spying? Consider all angles. It’ll give you a better view..."This is the view our staff see as they enter MI5 HQ in Thames House, London. Behind these pods lie some of the UK’s best kept secrets."
While the Secret Service won't be posting all its secrets online due to a need to operate undetected in dangerous environments, its director general said "the other half of the dilemma is that MI5’s ability to keep the country safe and resilient also depends on our reaching out to others who can help us, and whom we in turn can help".
Writing in the Daily Telegraph, Ken McCallum continued: “We owe it to the public to be constantly striving to learn and improve; and in our fast-moving world, with technology advancing at incredible speed, it would be dangerous vanity to imagine MI5 can build all the capabilities it needs inside its own bubble.”
Glasgow-raised Mr McCallum expressed hope the service being on Instagram would allow it to recruit Britain’s best and brightest, regardless of background.
“We must get past whatever Martini-drinking stereotypes may be lingering by conveying a bit more of what today’s MI5 is actually like, so that people don’t rule themselves out based on perceived barriers such as socio-economic background, ethnicity, sexuality, gender, disability, or which part of the country they happen to have been born in,” he wrote.
It comes after Mr McCallum said in his first media engagement in October that he wants MI5 to “open up and reach out in new ways”, particularly in communicating with younger audiences.
“Much of what we do needs to remain invisible, but what we are doesn’t have to be,” he said.
“In fact, opening up is key to our future success.”
The agency follows GCHQ, which joined the Facebook-owned social network in October 2018.