The 26-year-old won the Premier League and FA Cup during his time at Manchester City, and also has 11 senior England caps to his name.
However, given the Black Cats' disastrous start to the Premier League season and ongoing relegation battle, victory over City at Wembley on Sunday would be little short of remarkable.
Johnson said: "Especially the way the season has gone and playing City, it would probably be the highlight of my career, especially with Sunderland.
"When I was at City winning trophies and things, it was half-expected. But at Sunderland, no-one has even given us a chance of winning anything, so to bring that back would be massive."
Johnson made a £10m switch from City to the Stadium of Light in August 2012 after falling out of favour with then manager Roberto Mancini.
Although it took the Easington-born midfielder time to settle on Wearside, he has rediscovered his best form in recent months.
Johnson is not sure he has anything left to prove to anyone at City, but says that will make little difference to his approach.
"I don't know, it's been a while now since I left," he said. "But regardless of who you are playing for, as a player, you would always want to prove something in the final anyway whether you are the best player in the world or someone in his first final.
"You still want to be the best player on the pitch or the best player you can be, and I will be no different whether it's against City or anyone else."
Watch Manchester City v Sunderland on Sky Sports 1HD - coverage begins on Sunday from 12.30pm.