I have a similar question about that, I think I know the answer but just to be sure:¨
ESTA valid until May 12th 2016
Connecting in DFW to Mexico on May 10th 2016 -> ok
Connecting in MIA to LHR on the return on May 15th 2016 -> since I am not staying in the US, do I need to renew my ESTA? I guess yes since we always need to clear immigration when connecting in the US, but not sure about that.