Who is responsible for withholding the employees income tax?​

An employer's federal payroll tax responsibilities include withholding from an employee's compensation and paying an employer's contribution for Social Security and Medicare taxes under the Federal Insurance Contributions Act (FICA).

Employers have numerous payroll tax withholding and payment obligations. Of the utmost importance is the proper payment of what are commonly known as FICA taxes. FICA taxes are somewhat unique in that there is required withholding from an employee's wages as well as an employer's portion of the taxes that must be paid.

The Federal Insurance Contributions Act (FICA) is the federal law requiring you to withhold three separate taxes from the wages you pay your employees. FICA is comprised of the following taxes:

6.2 percent Social Security tax;
1.45 percent Medicare tax (the “regular” Medicare tax); and
Since 2013, a 0.9 percent Medicare surtax when the employee earns over $200,000.
You must withhold these amounts from an employee's wages.

The law also requires you to pay the employer's portion of two of these taxes:

6.2 percent Social Security tax
1.45 percent Medicare tax (the “regular” Medicare tax).
As you can see, the employer’s portion for the Social security tax and the regular Medicare tax is the same amount that you're required to withhold from your employees' wages. (Different rules apply for employees who receive tips.) There is no employer portion for the 0.9 percent Medicare surtax on high-earning employees.

In other words, you withhold a 6.2 percent Social Security tax from your employee’s wages and you pay an additional 6.2 percent as your employer share of the tax (6.2 employee portion + 6.2 employer portion = 12.4 percent total). Also, you withhold a 1.45 percent Medicare tax from your employee’s wages and you pay an additional 1.45 percent as your employer share (1.45 employee portion + 1.45 employer portion = 2.9 percent total). The total of all four portions is 15.3 percent (6.2 percent employee portion of Social Security + 6.2 percent employer portion of Social Security + 1.45 percent employee portion of Medicare + 1.45 percent employer portion of Medicare = 15.3 percent).

Unlike the other FICA taxes, the 0.9 percent Medicare surtax is imposed on the employee portion only. There is no employer match for the Medicare surtax (also called the Additional Medicare Tax). You withhold this 0.9 percent tax from employee wages and you do not pay an employer’s portion. Also, unlike the other FICA taxes, you withhold the 0.9 percent Medicare surtax only to the extent that wages paid to an employee exceed $200,000 in a calendar year. You begin withholding the surtax in the pay period in which you pay wages in excess of this $200,000 “floor” to an employee and you continue to withhold it each pay period until the end of the calendar year.