Stallings has replaced England's Roger Chapman in the field for the year's second major championship after the 2012 US Senior Open champion was forced to withdraw with a shoulder injury.
Stallings, who was fourth in the Memorial Tournament at Muirfield Village on Sunday, had been the first alternate from sectional qualifying in Memphis after shooting rounds of 70 and 69 at Colonial Country Club on Monday.
The top 60 in the world rankings on Sunday evening qualify for the US Open, although American Kyle Stanley is the only player currently inside the top 60 who is not already exempt.
Stanley's third place at the Memorial moved him from 90th to 59th in the standings, but he is not playing in Memphis and will face an anxious wait to see if he is overtaken.
Austria's Bernd Wiesberger (61st) is trying to qualify for his US Open debut and is the defending champion at this week's Lyoness Open on the European Tour.
World number 67 Jimmy Walker and number 68 Charles Howell are the highest-ranked players at the FedEx St Jude Classic who are not already exempt for Merion.
A total of nine major champions are in the field, including Phil Mickelson, Padraig Harrington and Trevor Immelman, while defending champion Dustin Johnson will be hoping to shake off a lingering back injury at TPC Southwind.
American Billy Horschel made his PGA Tour debut in the event in 2009 and has promised to continue supporting the event for as long as possible.
"They were great to offer me a sponsor's exemption, so I feel like I have to pay them back," said Horschel, who was missed just one cut all season and saw a run of four straight top-10 finishes culminate in his maiden victory in New Orleans in April.
"This is a great tournament, on a great golf course and the tournament does such a great job for St Jude (Children's Research Hospital). It's a tournament I'll play for years and years down the road. I don't see myself not being here."
As for his performances this season, the world number 53 feels they were long overdue and is not ready to settle for simply being a successful PGA Tour professional.
"I always knew I had the game," Horschel said. "I feel like I'm a really good player. I feel like I'm one of the top players out here, at least in my mind.
"If you look at my record, obviously I don't have the record of a Dustin (Johnson) or Tiger (Woods) or Phil (Mickelson) or Webb (Simpson). But I think in due time I, hopefully, will have what they have and take it to another level. And it's pretty cool to be where I am right now."