I am a Jordanian citizen living in Mexico. I will be traveling from Mexico to Amman, Jordan. Do I need a transit visa for CDG or LHR? I have less than a 24 hour layover.
You won't need a visa for a brief airside transit at either airport (however, I am not sure about the maximum duration for the layover, if any). For Heathrow, see gov.uk. For Charles-de-Gaulle, the rules are detailed in Do I need a visa to transit (or layover) in the Schengen area?
The reason for the difference between France and Germany you noted in the comments is that Jordan is not in the Schengen-wide list of countries whose citizens require an “airport transit visa” (they do in any case require a regular Schengen visa to enter any Schengen country). But Schengen member states can also add countries to that list (that's one of the few provisions that allow different Schengen member states to apply slightly different rules). According to the EU Commission website, Germany and the Czech Republic (but not France) do require airport transit visas for Jordanian citizens.