Haven’t found many details on the so-called “Tex-Mex mile” but it does sound like an interesting concept (and the series of restaurants is right by my place).

Here’s the best definition I’ve found so far (as part of a description of Jovita’s):

a one mile stretch of South First Street that has 11 Mexican Restaurants, a Mexican Bakery, Mexican Curios, Bar BQ, and a Funeral Home, just in case things get a little too spicy! 

Honky Tonk Texas, USA – Austin Restaurants

Another description, found in a mailing-list thread:

Tex-Mex Mile has a variety of inexpensive dining options, not all of which are nuclear hot
Tex-Mex Mile is S. 1st street from the river to Oltort, parallel to S. Congress, 2 blocks west

Comments on the following blog post mention a few places in connection with the “Tex-Mex mile” expression:

In The Pink Texas » Blog Archive » Holy Guacamole 

While they don’t use the same expression, food reviewers at the Austin Chronicle have described a stretch of South 1st in terms of its Mexican and Tex-Mex restaurants. That was a few years ago and it’s quite possible that the “Tex-Mex mile” expression was coined later to describe pretty much the same series of restaurants. Included in the Chronicle‘s coverage was a convenient map.

So, according to the Austin Chronicle, we had (going South on S 1st Street):

1. El Mercado
2. Jovita’s Cantina
3. Mercedes Martinez 
4. La Reyna
5. La Mexicana bakery
6. Polvo’s
7. Little Mexico
8. Tex-Mex Bar-B-Q
9. El Borrego de Oro (now on South Congress)
10. Taqueria Arandas #5
11. El Nopalito