I am using a lenovo thinkpad for work, can i use a mac monitor as an external display?

Which Apple monitor do you intend to use? If you mean can you use an iMac as an external display to the Lenovo, the answer is no. Macs are computers, they are not external displays for other devices.

you can do it if you use remote desktop, TeamViewer or other vnc setups, but of cause being across the network it may not perform as well as if one connected a real monitor to the thinkpad