The First Catalog of Long-Term Transient Sources detected by Fermi Large Area Telescope (LAT) comprises of 142 unique, transient sources that were detected on monthly time intervals during Fermi-LAT operations. The monthly time scale allows to identify transient and variable sources that were not yet reported in other Fermi-LAT catalogs. The monthly datasets were analyzed using a wavelet-based source detection algorithm that provided the candidate new transient sources. The search was limited to the extragalactic regions of the sky to avoid the dominance of the Galactic diffuse emission at low Galactic latitudes. The transient candidates were then analyzed using the standard Fermi-LAT maximum likelihood analysis method. All sources detected with a statistical significance above 4 sigma in at least one monthly bin were listed in the final catalog. This database table was ingested by the HEASARC in September 2021 and is based upon file downloaded from the Space Science Data Center (SSDC) Fermi-1FLT <a href="https://www.ssdc.asi.it/fermi1flt/">website</a>. This is a service provided by NASA HEASARC .