https://help.cbp.gov/s/article/Article-751?language=en_US
I'm pretty sure to MEXICO this is 100% possible, but for Canada we will find out this July 4th. Wish us luck
Traveling to other countries while in the United States on a B1 or B2 visa
Visitor visas are nonimmigrant visas for persons who want to enter the United States temporarily for business (visa category B-1), for tourism (visa category B-2), or for a combination of both purposes (B-1/B-2). During your visit to the United States, you may visit Canada or Mexico for up to 30 days and re-enter the United State, if you re-enter within the timeframe indicated on the Form I-94 that you received when you first entered.
For instance, if you arrived in the United States on July 10th with a B2 visitor visa, and on or after December 10th you decide you want to visit Canada or Mexico, you must keep in mind that the six-month period is up on January 10 and you will have to depart from the United States on that same day to avoid an "overstay" (unless you applied for an extension of stay).
Note: The six-month period is computer-generated starting on the day you arrive in the United States. It is recommended that you do not wait until the last day of your six-month stay to make travel plans because unexpected emergencies may occur.
If you visited other countries such as England or Costa Rica and then returned to the United States, your re-entry will be a new admission rather than a re-entry from a contiguous country during your initial visit, and the admission inspection may be more strenuous.
The CBP officer inspecting you will want evidence that you intend to go back home to your country of citizenship to live as opposed to returning again and again to the United States after visits to other countries. Do not forget, a B1 or B2 visa allows you to travel to the United States to visit. If the CBP officer suspects that you are attempting to be a de facto resident, you will be refused entry.
Re-entry, depends on your ongoing eligibility. If you have been arrested or committed an illegal act which has resulted in a warrant for you since the visa was issued, you may be refused entry.
For more information about B-1 and B-2 visas, please visit the Department of State-Bureau of Consular Affairs web site.
Article Number
000001422