Final answer:
Mussorgsky's career included both military and civil service roles, but he is most renowned for his musical compositions. False
Step-by-step explanation:
Mussorgsky's career was indeed more diverse than just composing music. He initially started his career in the military and then moved on to civil service. While he is primarily known for his contributions to music and as one of the Russian composers identified as the 'Mighty Handful,' his involvement in the military and civil services were significant aspects of his life. However, it is his musical composition work, particularly pieces like 'Pictures at an Exhibition' and 'Night on Bald Mountain', that have left a lasting impact and for which he is widely celebrated today.