The rural way of life was still dominant in most of Westchester in the decades after the Civil War. People led quiet, hard-working lives on their farms or in their villages, their normal routine relieved only by occasional visits to the larger towns. But change was in the air, and by 1914 the lifestyles in Westchester had changed so completely that an 1865 resident would have felt completely out of place. By then railroads, trolleys, automobiles, and paved roads had ended the isolation of Westchester's rugged countryside and opened up the county.

The county shared America's economic prosperity between the Civil War and World War I. Factories and suburbs developed along the Hudson River and in the interior south of White Plains. Thousands of Italians and eastern Europeans came to Westchester in a massive wave of immigration, to build railroads, dams, and mansions and to work in factories.

A growing middle class discovered the joys of leisure time. They took "the cars" to boardinghouses and hotels for vacations in the Westchester countryside. Many liked the area and bought gracious shingle and stone homes in the communities built by developers along the railroad lines. By the end of World War I, southern Westchester was an area of towns and growing cities; farms and villages were mostly located north of White Plains. The population had grown from about 100,000 in 1865, to almost 350,000 in 1920, an increase of 150 percent.

The New York Central's Putnam and Harlem divisions brought rail service into the north central region of Westchester. No longer did farmers have to make jolting trips by wagon to reach their markets. Once perishable milk products could reach New York City in a few hours by train, dairy farming became big business.

The railroads often determined whether a town grew or declined. In North Salem, a large Methodist church was built in anticipation of the growth expected when the railroad came through. But instead, the station was built at Purdys, a few miles to the west, and the church sat half-empty on Sundays. Three unsuccessful attempts were made to bring the railroad to Pound Ridge, and, when Somers voted against having the railroad come through the town, manufacturers, other businesses, and the Farmers and Drovers Bank all moved to Mount Kisco. By 1895 Mount Kisco, which had consisted of only four buildings in 1847, was a town bustling with factories and summer hotels. In both Chappaqua and Yorktown, the town centers shifted as businesses and stores relocated to be near the train station.

It was not long after the old Croton Dam was completed in 1842 that New York City made plans to build more dams and reservoirs in Westchester. Between the 1880s and the 1920s the Kensico, Croton, and Catskill water systems were constructed. The flooding of thousands of acres for these reservoirs created considerable dislocations in many towns north of White Plains. The building of the New Croton Dam and its reservoirs, for instance, forced the complete relocation of the village of Katonah to higher ground and the construction of thirty-two miles of new roads and nineteen new bridges. Legal proceedings on 600 land condemnations for the New Croton Dam dragged on for thirteen years. Many factories and mills, which lost their waterpower when streams were diverted for the water system, had to relocate or went out of business. In North Salem, the hamlet of Purdys was moved, and 5 percent of the town was inundated, including hundreds of prime acres of dairy land.  

The Kensico Water System was constructed in the 1880s. It included the Kensico Dam and the Byram Lake Dam; and it drew water from Little and Big Rye ponds and Wampus Lake. More water was brought to New York through the Croton Water System, developed between 1892 and 1907, which included the Titicus Dam, Muscoot Dam, Amawalk Reservoir, and a larger Croton Dam. 

The Kensico Dam
After New York incorporated the boroughs of the Bronx, Brooklyn, Queens, and Staten Island in 1898, the water supply had to be increased again. Plans for the Catskill Water System were approved by the state of New York, and between 1906 and 1915 more dams and reservoirs were built to bring an additional 500 million gallons of water a day to New York City. Water from the new reservoirs west of the Hudson had to be carried through Westchester by way of a new aqueduct which led to the new reservoir created behind a larger Kensico Dam, completed in 1915. By 1924 New York City owned 18,320 acres of land in fourteen Westchester towns. And still New York thirsted. The Delaware Water System, completed in 1944, increased New York's water supply by 800 million gallons a day from four reservoirs on the south slope of the Catskill Mountains.

In the last half of the nineteenth century, Westchester's proximity to New York City, its transportation systems, and its available labor force attracted many manufacturing concerns, particularly along the Hudson River. Peekskill and Croton continued to be centers for the iron industry. Some factories established at this time were Standard Oilcloth (later Standard Textile Products) in Buchanan; Mobile Company of America (later the General Motors Assembly Plant) in North Tarrytown; Lord and Burnham Greenhouse Manufacturing Company in Irvington; Hudson River Brewing Company in Dobbs Ferry; Zinsser Chemical, Hastings Pavement, and National Conduit (later Anaconda) Manufacturing Company in Hastings; and Otis Elevator and Alexander Smith Carpet Company in Yonkers.  

In this period, more and more people were anxious to move out of New York City to the suburbs. Communities were laid out in southern Westchester where farms had stood a generation earlier. As the population grew, villages and towns became incorporated, and taxes were collected to provide necessary public services, such as paved roads, sewers, fire companies, and police forces. Horse-drawn trolley lines were established throughout the county in the 1880s and were electrified by 1890. Steamboats continued to do a brisk freight and passenger business on the Hudson River and Long Island Sound right up to World War I. For twenty-five cents one could take a day trip on a Starin steamboat from New York to the internationally-famous Starin's Glen Island Resort in New Rochelle. 

People who were freed from the seven-day-a-week commitment to farmwork began to participate in all kinds of sports and recreational activities. Tennis, golf, baseball, canoeing, and bicycling grew in popularity. Wintertime activities included sleigh rides and tobogganing. In the days before ice-cutters, the Hudson River often froze from bank to bank, and hundreds enjoyed skating and ice boating on its surface. In the summer, vacationers flocked to the beaches of New Rochelle and Rye or to the lakes and hills of northern Westchester. The wealthy joined exclusive clubs such as the Larchmont and American Yacht clubs to participate in the new sport of ocean sailboat racing. Automobile clubs were formed for family outings, and it was not long before the first automobile races were run. In 1908 Walter Law staged the American International Race for Stock Cars. Thousands of spectators lined the thirty-five-mile course to watch twenty-two drivers careen along unpaved roads for a $10,000 prize.  

The period after 1865 brought vast fortunes to a new class of entrepreneurs in the New York area. Many built large estates in Westchester County. Colonial, Chateauesque, and Renaissance Revival houses were built on the hills overlooking the Hudson River and Long Island Sound. Perhaps the most fantastic of all were the homes built to resemble castles, such as Lyndhurst in Tarrytown, Ophir Hall in Purchase, Leland Castle in New Rochelle, and Carroll-cliff (now Axe Castle) in Tarrytown. In northern Westchester, magnificent mansions were built on the hilltops of New Castle, Bedford, and North Castle.

Lyndhurst in Tarrytown
None of the estates, however, matched the grandeur and scope of the Rockefeller estates. The largest mansion in the county was the 204-room Rockwood Hall, built by William Rockefeller in 1887 on 1,000 acres overlooking the Hudson River in North Tarrytown and Mount Pleasant. William's brother, John D. Rockefeller, completed his mansion and gardens on 3,500 acres in Pocantico Hills in 1913. His son raised his family of six children there and built a million-dollar recreation hall that included a bowling alley, squash court, tennis court, and swimming pool. John D., Sr., enjoyed playing golf daily on his own course built on the grounds, and when the Putnam railroad disturbed his peace, he persuaded the railroad to move its tracks five miles away.

In the years around World War I, Westchester joined the suffragette movement with vigor. By 1915 Westchester Life of To-Day reported that 20,000 women were enrolled in the suffrage cause, which had 102 clubs organized by assembly and election districts. In 1917 the New York legislature voted to give women the right to vote in state affairs, and in August 1920, Westchester's suffragettes celebrated the successful passage of the Nineteenth Amendment to the Constitution.

In 1914 Westchester residents anxiously watched the beginning of World War I, and when the United States joined the conflict, many Westchester men stepped forward to take part. Within a month, for example, twenty-eight Dobbs Ferry men had enlisted in the armed services. Home Defense leagues and Liberty Loan committees were formed in every town; women joined the Red Cross, farmed community gardens, and operated canteens. The village of Scarsdale prepared for civilian unrest by purchasing twenty riot guns and surveyed homes for guns and aliens. Thursdays and Sundays were designated as "lightless nights," to save coal for the war emergency.

For army units assigned to guard the dams and aqueducts, however, duty was not always arduous. "We're living like millionaires up here," stated Corporal Gerard of Company L's Tenth Regiment, which had been assigned to Shaft Nine of the aqueduct on the Rockefeller estate. John D. Rockefeller supplied the soldiers daily with eggs, sandwiches, coffee, and a cooking stove.  

As the war drew to a close, Westchester celebrated Armistice Day with parades and church services. In Dobbs Ferry, Miss Masters marched the girls of the Masters School to the Presbyterian Church, where, after a brief service, they paraded through the village singing "Onward Christian Soldiers."