In 2016, Flintshire was transferred to the United States and entered the stable of Chad C. Brown.
On his first appearance for his new trainer, he contested the Grade I Manhattan Handicap at Belmont Park and started favourite in an eleven-runner field which included the 2015 American Champion Male Turf Horse, Big Blue Kitten.
Ridden by Javier Castellano, he tracked a slow place, then kicked free in the stretch.
He won in a time of 1:58.92 for 1.25-mile (2,010 m).

On 30 July, Flintshire was the 1-9 favorite in the Bowling Green Handicap at Saratoga, facing three rivals.
Grand Tito set a "glacial" place of 1:19.4 for the first 6 furlongs in the ​1 ³⁄₈-mile race.
Flintshire was close behind the leader but boxed by the other two horses against the rail.
As they entered the stretch, Flintshire dropped back and started to swing around the other horses, then had to adjust his path again.
Finally in the clear, he accelerated rapidly and won by ​³⁄₄ lengths.

On 27 August, Flintshire won the Sword Dancer Stakes for the second year in a row.
To prevent a repetition of the problems experienced in the Bowling Green, Juddmonte also entered Inordinate as a "rabbit" to ensure a fast pace.
As they entered the stretch, Flintshire was moving well but looked trapped on the rail.
Inordinate's jockey pulled the horse wide, causing another horse to check while opening a hole for Flintshire to accelerate through.
Flintshire drew off to win by ​1 ³⁄₄ in an excellent time of 2:23.45, just ​¹⁄₅ off the stakes record, having run the last quarter mile in 22 seconds.
A claim of foul against Inordinate was disallowed, but the stewards did speak with Brown and the jockeys involved.

On 3 October, Flintshire finished second in the Joe Hirsch Turf Classic on yielding turf, unable to make up enough ground on the front-running Ectot. "
We knew the risk going into the race running on soft ground.
He has a poor record on it", said Brown. "
I didn't see another option; I didn't feel comfortable going into the Breeders' Cup without a prep.
So, we gave it a shot and it didn't work out."

On 5 November, Flintshire was made the favorite for the $4 million Breeders' Cup Turf.
The other nine runners included Highland Reel (King George), Found (Arc, 2015 Breeders' Cup Turf), Ectot, Ashleyluvssugar (Del Mar Handicap), Da Big Hoss (American St. Leger Stakes), Mondialiste (Arlington Million), Ulysses (an improving three-year-old from Europe) and the veteran Twilight Eclipse (Man o' War Stakes).
Unexpectedly, Highland Reel took the lead soon after the start and set a steady pace before kicking seven lengths clear of his opponents at half way.
Flintshire made a late move but had too much ground to make up, finishing ​1 ³⁄₄ lengths back, with Found staying on to take third place. "
Unfortunately, the way it worked out, it was unfortunate for my horse", said Castellano. "
Usually you don’t see the European horses run so aggressively, open up, and carry their speed — especially at 1 1/2 miles — but it worked out great for him."
In 2016, Flintshire was transferred to the United States and entered the stable of Chad C. Brown. On his first appearance for his new trainer, he contested the Grade I Manhattan Handicap at Belmont Park and started favourite in an eleven-runner field which included the 2015 American Champion Male Turf Horse, Big Blue Kitten. Ridden by Javier Castellano, he tracked a slow place, then kicked free in the stretch. He won in a time of 1:58.92 for 1.25-mile (2,010 m).
On 30 July, Flintshire was the 1-9 favorite in the Bowling Green Handicap at Saratoga, facing three rivals. Grand Tito set a "glacial" place of 1:19.4 for the first 6 furlongs in the ​1 ³⁄₈-mile race. Flintshire was close behind the leader but boxed by the other two horses against the rail. As they entered the stretch, Flintshire dropped back and started to swing around the other horses, then had to adjust his path again. Finally in the clear, he accelerated rapidly and won by ​³⁄₄ lengths.
On 27 August, Flintshire won the Sword Dancer Stakes for the second year in a row. To prevent a repetition of the problems experienced in the Bowling Green, Juddmonte also entered Inordinate as a "rabbit" to ensure a fast pace. As they entered the stretch, Flintshire was moving well but looked trapped on the rail. Inordinate's jockey pulled the horse wide, causing another horse to check while opening a hole for Flintshire to accelerate through. Flintshire drew off to win by ​1 ³⁄₄ in an excellent time of 2:23.45, just ​¹⁄₅ off the stakes record, having run the last quarter mile in 22 seconds. A claim of foul against Inordinate was disallowed, but the stewards did speak with Brown and the jockeys involved.
On 3 October, Flintshire finished second in the Joe Hirsch Turf Classic on yielding turf, unable to make up enough ground on the front-running Ectot. "We knew the risk going into the race running on soft ground. He has a poor record on it", said Brown. "I didn't see another option; I didn't feel comfortable going into the Breeders' Cup without a prep. So, we gave it a shot and it didn't work out."
On 5 November, Flintshire was made the favorite for the $4 million Breeders' Cup Turf. The other nine runners included Highland Reel (King George), Found (Arc, 2015 Breeders' Cup Turf), Ectot, Ashleyluvssugar (Del Mar Handicap), Da Big Hoss (American St. Leger Stakes), Mondialiste (Arlington Million), Ulysses (an improving three-year-old from Europe) and the veteran Twilight Eclipse (Man o' War Stakes). Unexpectedly, Highland Reel took the lead soon after the start and set a steady pace before kicking seven lengths clear of his opponents at half way. Flintshire made a late move but had too much ground to make up, finishing ​1 ³⁄₄ lengths back, with Found staying on to take third place. "Unfortunately, the way it worked out, it was unfortunate for my horse", said Castellano. "Usually you don’t see the European horses run so aggressively, open up, and carry their speed — especially at 1 1/2 miles — but it worked out great for him."
Flintshire (horse)