MBP Mini-DisplayPort to USB-C Monitor I have an "old" MacBook Pro (MBP) that has a Mini-DisplayPort connector.

I have a newer MBP with 4 USB-C connectors that I use with an LG UltraFine 5K Monitor (aka. LG 27MD5KA-B).

I wanted to know if through an adapter, I could connect the old MBP from its Mini-DisplayPort to the USB-C of the monitor and have a video signal.

This Apple StackExchange question does not really answer my question because it is about USB-A to USB-C. This Apple KB (section System requirements and compatible Mac models, paragraphe before the last one) says no, but does not specify Mini-Display Port. Finally, the answer to this Apple StackExchange question says no but then says that "Thunderbolt is a protocol for combining PCI express data with DisplayPort video protocols" (this web page explains the differences and similarities between Thunderbolt 3 and USB-C) and then says "DisplayPort embedded over USB 3.1/Type-C".

I am therefore a bit confused: Mini-DisplayPort (a video protocol) uses a Thunderbolt connector and DisplayPort is embedded over USB 3-1/Type-C…

My question: What happens if I buy a cable like this and connect it between my old MBP and my LG UltraFine 5K Monitor?
That cable won’t work.

First, the monitor doesn’t have a DisplayPort input nor does it have a USB upstream port.  It has a Thunderbolt 3 port that uses the USB Type C connector.  The USB-C down stream ports are supplied by the Thunderbolt connection. 

Secondly, even though TB is backward compatible, it’s one way.  So, a laptop with a TB 3 port can work with a TB 1 or 2 device, but a TB 3 device can’t work with a TB 1/2 laptop port.  The adapter can’t add capabilities to a machine that doesn’t already have.