The Leicester midfielder was named in the 26-man squad that will go to Qatar after an impressive run of form this season.
There were large question marks over his place having played just 36 minutes for the Three Lions under Southgate, with stories as late as Wednesday night suggesting he was not in the squad, but he is on the plane.
Foxes boss Brendan Rodgers revealed that Maddison had to phone Southgate back from his office to get the news he was going.
"He was delighted, he had a missed call from Gareth just before we were ready to go out training and I told him to pop into my office and take the call privately," Rodgers explained.
"He obviously spoke to Gareth and he came out full of the joy of the world.
"It's private what James said, it was just happiness as well because I am sure he had all sorts of thoughts going through his mind.
"Sometimes how it was painted it looked like it was swinging more against him, if you listened to stories. In fairness to Gareth he has been great, he looked at it with logic and saw a player that can really help the squad.
"He called to stabilise James because there were stories overnight that sounded like he wasn't going to make it. So really good management with Gareth to stabilise it.
"It is wonderful news for James and his family and everyone here at Leicester. I am also delighted for Gareth as well from a coaching perspective to have that talent available in what is such a great tournament.
"His level of consistency and the work ethic and the talent and what he has proven over the last few years has swung that decision.
"He is a fantastic player and one of the best players in the Premier League. I am really delighted for him and also his team-mates because without them he wouldn't have been able to play at the level he has."