It is advised that you do this on a PC.
1. Export: You can export contacts from your Mi account to a Vcard (Vcf) file. This can be done by logging into your Mi Account at - https://i.mi.com/
Choose Export Contacts > Export and you will get the Vcard file to download.
2. Import: You can then import those to your Google contacts by logging into your Google account at https://contacts.google.com
Choose More > Import at from the left sidebar. Then click on CSV or vCard file and select the Vcard file which you downloaded from Mi contacts.
Your contacts will be imported. For the future, change where to save the contact to "Google account" while adding a new contact.