Ridden by Alex Solis, the 5-year-old stormed home to hit the wire in 1:46.64 for the 11/8 miles.
Hymn Book, the 5-2 second choice, finished second -- a length ahead of favorite Rodman.
The victory was the fourth in nine lifetime starts for Flat Out, who is trained by Charles "Scooter" Dickey.
Flat Out paid $29.20 to win.
Dominus beat 4-5 favorite Adios Charlie by 13/4 lengths in the $150,000 Dwyer Stakes for 3-year-olds. He paid $7.80 to win with Julien Leparoux aboard for trainer Steve Asmussen.
Teaks North takes UN
Teaks North held off two rivals in the stretch to win the $750,000 United Nations Stakes at Monmouth.
Trained by Justin Sallusto and ridden by Eddie Castro, the 4-year-old gelding ran the 1 3/8 miles on a firm turf course in 2:13.28 and paid $16 to win. The win in the Grade I race was the seventh in 17 career starts for Teaks North, who has now earned $976,964.
Earlier in the card, Kensei held off Soaring Empire in the deep stretch to win the $200,000 Salvator Mile by three-quarters of a length. Ridden by Edgar Prado and trained by Asmussen, the win in the Grade III race was the fifth in 17 career starts for the 5-year-old.
Lopez gets 4,000th win
Jockey Chuck C. Lopez scored his 4,000th win in the first race at Monmouth aboard Duffy's Tavern. The 50-year-old Brooklyn native was the top apprentice at Monmouth in 1979, the same year he scored his first career win with Foolish Tracy at Keystone.