You appears to be using the Javascript version of the Google Places API. Let me know if I've guessed incorrectly!
All you need to do is add ®ion=US
when you load the Google Maps API. E.g.:
<script src="https://maps.googleapis.com/maps/api/js?key=YOUR_API_KEY&libraries=places®ion=US">
Note that this will still show the country for Autocomplete predictions outside of the United States (which helps disambiguate "Paris, TX" from "Paris, France").
See the section on Region localization in the Google Maps APIs documentation for more details on this parameter.
If you need even more control over the appearance, you can use the AutocompleteService
programmatic API to build your own UI, rather than using the Autocomplete
control.