you say mirroring was turned off or disabled, but that doesn't mean it was done correctly. If it's not removed from the source, and you don't have a destination, the log will continue to grow.

When you query sys.databases, what is returned for the log_reuse_wait_desc? It should be a number.

If this is 0, then either you have an open transaction, or you are not running log backups as you think you are. Double check everything is correctly configured.

you could try switching to simple mode, then back to full, then take a full backup and shrink the log (DBCC SHRINKFILE). then monitor to see what happens.