Longstreet was an information systems specialist as well as an historian. He was particularly knowledgeable about the period of Earth's early to mid 21st century. He was also cross-trained to serve as part of the bridge crew, including the helm. (TOS novels: Across the Universe, Garth of Izar)
In 2267, when the Enterprise first encountered the 21st century Earth colony ship Stephen Hawking, Longstreet created a cultural and historical primer to help the travelers acclimate to the 23rd century, and worked with Nyota Uhura to prepare a presentation for the crew on Merope IV, a likely site for them to disembark. He also beamed aboard the vessel with Spock and Montgomery Scott to confirm that it contained a hidden thermonuclear bomb.
While in orbit of Merope IV, Longstreet and Uhura reviewed Hawking's personnel records with First Citizen Teressa Aliss in the city of New Ibadan. Longstreet and Pavel Chekov later joined several Hawking personnel to try to stop an approaching Meropean bio-mass from engulfing the city. Phaser stun proved ineffectual against the bio-mass, and the officers were beamed up after their ground vehicle became trapped.
In the aftermath of the bio-mass assault, Longstreet manned the communications station. He became dazed when the bio-mass sent a neural feedback pulse to the Enterprise, recovering quickly but with a lingering headache. When Spock beamed down to attempt to contact the planetary intelligence, Longstreet manned the sciences station and tracked Spock's coordinates. (TOS novel: Across the Universe)
In 2270, when the Enterprise returned Garth of Izar to Antos IV, Longstreet served as relief helmsman opposite navigator Enrico Carulli while Hikaru Sulu visited the planet. (TOS novel: Garth of Izar)