Transitioning mountain bike into city commuter bike In effort to transition my mountain bike to a city commuter- can I replace the mountain tires with thinner city tires without changing the bike wheels?

Short answer: Yes, you can.

The rims on most mountain bikes will support a tire as thin as 26 x 1 inch. 26 x 1.25 is virtually guaranteed on any mountain bike at all. (As long as it is a 26" wheel bike.")