Skyhawk 172/Cutlass 172
Air Traffic Control Fully Certified
Air Traffic Control Time Certified In Pos 1 (yrs) 1
Situational Awareness; Distraction
Conflict Ground Conflict; Less Severe; Deviation / Discrepancy - Procedural Clearance; Ground Incursion Runway
Air Traffic Control Issued New Clearance
Environment - Non Weather Related; Human Factors
I just took transfer for Local Control. Aircraft X was a full stop on their roll out on and were instructed to exit which was read back and the pilot exited. The pilot was then instructed to continue; turn left and hold short of a runway which was also read back. The hold short instruction was given due to another aircraft; Aircraft Y; who was about to touch down. Aircraft X was observed crossing the hold short line and continued across the runway. I tried to communicate to them about stopping immediately but was too late. Meanwhile Aircraft Y was airborne; between 100-150 feet and was flying over the runway intersection simultaneously. A direct overflight did not occur; but it was very close. Aircraft X was told about the hold short instruction and then issued a Brasher Warning.This occurred directly after I took over the position; although traffic was not complex or heavy having to adjust the position to my needs may have been a contributing factor to not seeing the incident sooner. Maintaining vigilance even during slow times is my personal recommendation.
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BJC Tower Controller and a GA pilot reported a runway incursion during taxi.