A federal income tax was imposed on several occasions in the 1800s. However, in response to a court case which determined that income from property was required to be imposed in proportion to states' population, Congress proposed the Sixteenth Amendment. Thus in 1913, the modern income tax system was born.
In 1913, 358,000 returns were filed which was 2% of all households. While the top tax rate was 7% on incomes above $500,000 ($10.9 million in 2010 dollars), the first $3,000 ($65,331 in 2010 dollars) was exempt from the the income tax for single persons.
In 1918, 4,425,000 returns were filed which was 20% of households. Now the exemption was $1,000 ($14,352 in 2010 dollars) while the top rate of 77% was now applied on income over $1,000,000 to pay for World War I ($14.3 million in 2010 dollars). There was high inflation during and right after the war so by the peak in 1923 almost 40% of households were being taxed due to bracket creep. This was fixed in 1925.
In 1942, 36,619,000 returns were filed and the exemption had been dropped to $500 for single persons ($6,613 in 2010 dollars). For the first time the number of income tax returns filed exceeded the number of households.
Data: Top Income Tax Rates; Tax Exemptions; Number of Tax Returns; Number of Households