By 1905, Mercer County's population was well over 100,000, with Trenton accounting for just over 96,000 people.

The turn of the century saw the automobile make horse-drawn carriages a thing of the past. The Mercer Automobile factory was born soon thereafter and flourished for years. Joining the still growing industrial powerhouse of Mercer County were professional sports teams to entertain the community.

Baseball teams were abundant and successful. Soccer and basketball also joined the area, as well as skating and ice-boating venues on the Delaware.

In 1910, New Jersey elected a new Governor. Woodrow Wilson, the former President of Princeton University, became an immediate success in New Jersey politics. Indeed, he was so good as his job, that the nation soon took notice, and in 1912, Woodrow Wilson became the 28th President of the United States.

Though not a Mercer story, it is important to note that President Wilson was re-elected in 1916 and soon entered the United States into World War I.
The entrance of America into World War I left Mercer County's industries to cope with a huge increase in demand for new materials while at the same time losing a large amount of the workforce who went to serve the country. The resulting demand of supplies increased the power and prestige of the county, while further making Mercer County an important place for the entire country.

Of course, World War I ended in 1917 with an allied victory over the armies of Kaiser Willhelm of Germany. The end of the war did not signal the end of the growth of Mercer, however. Following the war, Mercer County hosted numerous celebrations to enjoy the victory. Soon thereafter, the area also celebrated the anniversaries of such notable events as Washington's crossing of the Delaware, the anniversary of the adoption of the Star Spangled Banner, and even the birthday of George Washington.

It should also be noted that on October 27, 1924, the people of Mercer County and the surrounding regions celebrated the opening of the Mercer Airport, now the Trenton-Mercer Airport. Over 35,000 people witnessed Mercer's entrance into the air transportation industry.

Then came the Great Depression. And unfortunately, Mercer County too felt the sting of the economic problems. The once great and prosperous region, home of some of the greatest industries in America, was entering a time of confusion, despair, and lost hope. So while the beginning of the last century saw the birth and development of Mercer, the beginning of the 20th century gave Mercer a vision of war and depression. And like most areas in the country, Mercer County did not thrive again until the start of another Great War.