Bjc tower controller and a GA pilot reported a runway incursion during taxi.

Attributes
ACN	1557302
Time
Date	201807
Local Time Of Day	1801-2400
Place
Locale Reference	BJC.Airport
State Reference	CO
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Make Model Name	Skyhawk 172/Cutlass 172
Operating Under FAR Part	Part 91
Flight Phase	Taxi
Route In Use	None
Flight Plan	VFR
Aircraft 2
Operating Under FAR Part	Part 91
Flight Phase	Initial Climb Takeoff
Route In Use	None
Flight Plan	VFR
Person 1
Function	Local
Qualification	Air Traffic Control Fully Certified
Experience	Air Traffic Control Time Certified In Pos 1 (yrs) 1
Person 2
Function	Single Pilot Pilot Flying
Qualification	Flight Crew Instrument Flight Crew Multiengine Flight Crew Commercial
Experience	Flight Crew Last 90 Days 43 Flight Crew Total 536 Flight Crew Type 55
Events
Anomaly	Conflict Ground Conflict Less Severe Deviation - Procedural Clearance Ground Incursion Runway

Narrative:

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.

Original NASA ASRS Text

Title: BJC Tower Controller and a GA pilot reported a runway incursion during taxi.

Narrative: 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.

Data retrieved from NASA's ASRS site and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.