Flying to Nairobi from North America.. Any cheaper hubs to fly from?

Have you tried a search on a site like Kayak.com? You should get an idea pretty quickly.

If you are in a far-flung part of the Western Hemisphere, the critical factor is the cost of the flight that will connect you to the major US/Canadian hubs: Toronto, Montreal, New York and Washington. From there you have a choice of several airlines who compete fairly closely with each other. I flew from Toronto in September and the two best fares were Turkish and Ethiopian, actually within a dollar of each other! They are both Star Alliance members, so that could help with your connection from Mexico. KLM, though not as bargain-oriented as it used to be, sometimes offers deep discounts for very limited times so it is worth signing up for their promo emails. British Airways may also give you a decent fare.

British Airways and Kenya Airways never seem to give [I ]Me[/I] any decent fares...

Definitely look for flights out of Houston via Europe (Amsterdam can be good). Also, Delta flies Atlanta to JNB and partners with South African Airways, so you might find something from there. Both should have reasonable access from Cancun, especially Houston.

I would took a look at the fares of British Airways and Air France out of Cancún just in case they were decent enough as to avoid and extra flight leg. You could also look for fares out of Mexico DF with KLM or the already quoted BA and AF.

You can look for flights out of New York (good choice of airlines from New York to Nairobi changing planes on a third airport). In order to go from Cancún to New York, Jetblue could be a good option.

Another option you could research is a flight from Cancún to Europe (e.g.: XL Airways from Cancún to Paris) and a separate booking from Paris to Nairobi... Or a flight to Mombasa out of Brussels (Brussels is just a short train ride away from Paris) with Jetairfly...

SeaStarr, check the KLM website as soon as possible. They are offering Toronto-Nairobi return for $999 for departures between March 10 and April 7, 2014. Don't know if that fits your schedule. To qualify you have to book by Jan 7.

Cheapest fares I've found from the States to Kenya have been on Ethiopian (through Addis) or Qatar Airways (through Doha). If you can get a cheap flight from Cancun to DC, Ethiopian flies direct from there to Addis. Qatar has a few cities it flies from, including Chicago and NYC.

You may want to check out the NYTimes recent piece about searching for the best multileg flights: http://www.nytimes.com/2014/01/15/travel/how-to-choose-an-air-travel-search-site.html?_r=0