For the space flight mission MESSENGER, the Mercury Laser Altimeter (MLA) instrument required highly reliable optical fiber assemblies for the beam delivery system. A custom assembly was designed based on commercially available technologies to accommodate the requirements for the mission. Presented here are the results of environmental testing the MLA optical fiber assemblies. These assemblies consisted of W.L.Gore FLEX-LITETM cable with 200 micron core Polymicro Technologies optical fiber and the Diamond AVIMS connector kits. The assemblies were terminated to the NASA-STD-8739.5 in the Code 562 Advanced Photonics Interconnection Manufacturing Laboratory at NASA Goddard Space Flight Center. The technology validation methods that were used to characterize these assemblies for usage in a space flight environment have been established and well documented.[1-4] Only the tests that are known to bring out the failure modes typical to optical fiber assemblies have been performed to assess the ability of these assemblies to withstand the environmental parameters that have been established for Mercury Laser Altimeter. Testing involved vacuum, vibration, thermal and radiation exposure and the data with results of those characterization tests are included here. For the radiation characterization, both the 200 micron core FLEX-LITETM (FON1173) and the 300 micron core FLEX-LITETM (FON1174) were tested. For all other environmental tests only the 200 micron core FLEX-LITETM /AVIMs assembly was tested since it was expected that the results of such testing would not differ much with core size.