Mosley was a 23rd century Human man, a Starfleet security crewman serving aboard the USS Enterprise under the command of Captain James T. Kirk. (TOS novel: Devil World)
Biography
Mosley was close friends with fellow Crewman Doyle in the security division. People described him as an optimist, and he had a more pleasant demeanor than Doyle. Spock described him as a competent professional.
Mosley and Doyle accompanied the landing party to the planet Heartland. Kirk had Mosley join the team in his hut in the Danon village, while Doyle stayed with Spock's team. During a poker game that evening, Mosley lost every credit he owned to Doyle and Dazi.
After Doyle was found catatonic, Mosley realized the Danons kept the man's phaser. The next night, Hikaru Sulu and Mosley joined Albert Schang to fight through a horde of Danons within the Great Machine to rescue Kirk. Once back aboard the Enterprise, Kirk recommended all three for fleet citations. (TOS novel: Devil World)
When the Enterprise staged a raid against a Romulan space station in orbit of Levaeri V in 2275, Mosley was one of the hundred-odd crew people chosen to participate. (TOS novel: My Enemy, My Ally)