I had an electrician install a 20 amp circuit to replace the 15 amp the microwave was connected to. He ran the 12 gauge wire to the junction box in the attic, but not all the way to the outlet. This does not make sense to me, since the wire from the junction to the receptacle is rated only for 15 amp. The breaker he installed is 20 amps, so I'm concerned the 15 amp portion of the wire may get too hot & not trip the breaker. Is this a safe configuration? Appreciate any help.

You are correct in you concerns. All wiring on a 20 amp circuit must be 12 gauge. First check to insure that the cable from the junction box is 14 gauge. You want to make sure you are correct before faulting some one. Then call him back. This is a code violation and not a technical one. It poses real risk.

This electrician should know better.

I know what he will say, the amp load will never exceed the ampacity of the section of #14 wire, but still, it is just wrong.

Tell the electrician to check with the local electrical inspector, with you as part of the discussion, and see what he has to say about this.