Egan, a Republic of Ireland international, is now tied to the Premier League club until 2024 following talks with members of its hierarchy.
Chris Wilder, the United manager, described the development as a “reward” for the centre-half’s contribution towards his team’s remarkable progress in recent seasons - helping them win promotion from the Championship before challenging for a place in Europe and reaching the quarter-finals of the FA Cup last term.
Outlining his ambitions for the future, Egan said: “The last two years have probably been the most successful of my career and to get the chance to sign a new contract here is brilliant for me, and I’m really looking forward to the future. I need to thank everyone involved.”
“I’ve loved my time here. We won promotion in the first year and had a real good first season back in the Premier League, so now it’s about building on that and improving in the future.”
Egan, aged 27, was joined at United this week by goalkeeper Aaron Ramsdale - who has returned to South Yorkshire three years after leaving for AFC Bournemouth.
Wilder, who paid around £6m to sign Egan from Brentford, said: “It’s a reward for the influence John has on the team and at the football club. He’s been part and parcel of our climb and he’s taken his game to the next level.”