Thanks for pointing me in the right direction. The Excel ODBC driver makes some quirky assumptions indeed. I found I was able to change the column data type within my Excel spreadsheet to text explicitly and this solved the problem. Apparently the driver scans the first 8 rows and if a majority are numeric, it assigns the Double type.
I was able to do a Format Cells > Number > Text on the impacted columns and this forced the ODBC driver to send the right values to MySQL.
Here’s a Microsoft KB article that covers this exact issue: https://support.microsoft.com/en-us/kb/141284
Appreciate your assistance!