You don’t need to connect to a external monitor when the Raspberry Pi is connected to a network. You can use Virtual Network Computing by installing tightvnc on your Raspberry Pi and using the pre installed “screen sharing” on your Mac.
First you need to install tightvnc on your Pi:
pi@raspberrypi ~ $sudo apt-get install tightvncserver
and run it
pi@raspberrypi ~ $tightvncserver
If you have a Mac you need to go to the “Finder” and select “Go > Connect to Server…” or use the Shortcut “CMD + K”