[3] Belmont founded the Interborough Rapid Transit Company in 1902 to help finance the construction of and operate the first line of what is now the New York City Subway.

Another son, Raymond Belmont, served as an Infantry officer with the 78th Division in France and participated in Saint-Mihiel and Meuse-Argonne offensives.

His wife, Eleanor, also devoted much time to raising funds in aid of Belgian relief efforts and for the Red Cross, she made a number of trans-Atlantic trips as an inspector of United States Army camps.

The narrow 140-foot (43 m) width and shallow 25-foot (7.6 m) depth of the canal made navigation difficult, and tidal flows created dangerous currents, so many mariners continued to use the routes around the cape.

Belmont inherited Nursery Stud, a Thoroughbred breeding operation established in 1867 by his father at his 1,100-acre (4.5 km2) Babylon, New York, estate.

In 1905 he built Belmont Park racetrack in Elmont, on Long Island which operates to this day as the largest thoroughbred racing facility in the state.

In the year of its opening, the prestigious Belmont Stakes, inaugurated in 1867 and named in his father's honor, was transferred from the financially troubled Morris Park Racecourse.

[10] Belmont also had horses competing in England and in 1908 his American-bred colt Norman III won a British Classic Race, the 2,000 Guineas.

In addition to his Kentucky horse farm, in 1908 Belmont established Haras de Villers, a breeding operation near Foucarmont in Upper Normandy, France.

Following the cessation of racing in New York State as a result of the Hart–Agnew Law banning parimutuel betting, Belmont stood American stallions at Haras de Villers such as Flint Rock, Ethelbert, and the sire of Norman III, Octagon.

At his French farm, he bred notable horses such as Prix de Diane winner Qu'elle est Belle as well as Vulcain, one of the best three-year-olds of his generation in France.

His son Raymond owned Belray Farm near Middleburg, Virginia, where the Hall of Fame horse Colin lived out his final years, dying there in 1932 at the age of 27.

Under the control of planner Robert Moses, the estate was later expanded to 459 acres (1.86 km2) and turned into Belmont Lake State Park.

The mansion served as headquarters for the Long Island State Park Commission until 1935, when it was demolished to make way for the current building.

Two lines of pine trees that formerly surrounded the mansion's driveway are preserved in the median of the Southern State Parkway.