Apple's MacBook comes equipped with an LCD display that many users will find perfectly suitable for most applications. However, should you wish to attach a second monitor to the device for presentations or for a larger viewing area, the MacBook comes equipped with a video-out port. By default, when an external monitor is attached to the MacBook, it extends the desktop area; you can move your mouse and work freely between the two monitors. To change this behavior such that actions performed on the main display are mirrored on the external monitor, you must adjust the display settings located in the computer's System Preferences window.
Locate a video-out port on the side of your MacBook. The available ports will depend on your MacBook's model. For example, a MacBook Air has a Thunderbolt port, while a MacBook Pro with Retina display has two Thunderbolt 2 ports and an HDMI port.
Examine the video port on the external display, and then use the appropriate cable or adapter to connect your MacBook to the display. For example, if your display has a DVI port and your MacBook has a Thunderbolt port, use a Mini DisplayPort to DVI Adapter to connect the two devices. In this example, connect the Mini DisplayPort end of the adapter to your MacBook's Thunderbolt port, and then connect the other end to the display's DVI port.
Open the "Apple" menu, and then select "System Preferences."
Click the "Displays" icon, and then select the "Arrangements" tab.
Click the "Mirror Displays" check box in the lower left corner of the window.
Click the red button in the upper left corner to close the window.
Things You'll Need
Mini DisplayPort adapter
HDMI cable or adapter
If you have not reassigned your function keys to a different purpose, you may also activate monitor mirroring by pressing the MacBook's "F7" key.
You may connect a DVI, Dual-Link DVI or VGA monitor to your MacBook; however, use a Mini DisplayPort adapter designed for the monitor you intend to use.
You may purchase Mini DisplayPort adapters directly from Apple's website.
If you wish to connect an Apple display equipped with a ADC connector, use a DVI to ADC adapter in conjunction with a DVI to Mini DisplayPort adapter. You can purchase a DVI to ADC adapter directly from Apple's website.
If you have multiple monitors connected to your MacBook, but only wish to mirror to a single monitor, link the MacBook's display to that of the target monitor. While in the Arrangement tab of the Display window, click on the square representing your MacBook's display. Hold down the option key, drag the square to the one representing the target monitor, and then release.
If the display resolution on the external monitor is higher than that of the MacBook's LCD screen, the monitor will use the lower resolution while mirroring is active.
Information in this article applies to OS X Mavericks. Instructions may vary slightly or significantly with other versions of the program.