You can follow below steps

1. Run below script against db in mirroring on principle server.

DECLARE @DatabaseName VARCHAR(50);

SET @DatabaseName = 'Mirrored DB Name' --> Replace the mirror db name here.

SELECT name, recovery_model_desc, log_reuse_wait_desc

FROM sys.databases

WHERE name = @DatabaseName

2. If output column log_reuse_wait_desc is saying LOG_BACKUP then simply take multiple time log backup of mirror database. Mostly 3 continuous T-log backup would enable db to shrink the log file with following command.

DBCC Shrinkfile(2,1024)

Above steps would fix the issue.

Thanks,

Mahesh Shinde