Leslie was a 23rd century Starfleet officer. He served aboard the USS Enterprise between 2265 and 2268, during the historic five-year mission. By 2266, he held the rank of lieutenant.

Even though he was primarily assigned to the operations division, Lieutenant Leslie was a versatile officer who was cross-trained to serve in several departments as needed. In most of his duties on the Enterprise, he worked at the engineering station on the bridge, just next to the turbolift doors.

Under the command of Captain James T. Kirk, Leslie performed a variety of functions on the Enterprise, and the different Starfleet uniforms he wore reflected it. At the start of the five-year mission, he wore both a beige operations division uniform and a blue sciences division uniform. After this style was replaced by the newer one in 2266, he regularly wore the red operations division uniform whenever he performed the duties of security officer or engineer.

Occasionally, he sported the gold command division uniform whenever he was posted as the ship's navigator. He sometimes wore this uniform as he walked along the corridors of the Enterprise. also a blue one whenever he worked in sickbay or was assigned to the sciences division of a landing party.

Before stardate 2126.1, his uniform showed no rank insignia until he was promoted to the rank of lieutenant. With rare exceptions after this point, his uniform showed a full lieutenant stripe.

On two separate occasions, Leslie sat in the Enterprise command chair and performed the tasks of a duty officer.

The first time happened after he became a lieutenant on stardate 2126.1. He was the duty officer when Kirk and several other Enterprise officers safely escaped from Trelane of the planet Gothos. The second time happened on stardate 3088.7, when the starship investigated the cause of massive spatial disturbances on an iron-silica planet and encountered Lazarus.

Of all the duties he had on the Enterprise, Leslie was well known for his skills as an engineer. Even though he was occasionally assigned to the transporter room and main engineering, he was regularly posted as a bridge engineer on many missions.

He was at the engineering station when the Enterprise crew first encountered Harcourt Fenton Mudd, learned the fate of Doctor Roger Korby, and visited Miri's homeworld. Later that year, he witnessed Doctor Simon Van Gelder burst onto the bridge with a phaser.

The following year, he was at the same post when the Enterprise returned to Talos IV, pursued the Gorn ship, and orbited the Halkan homeworld. As the bridge engineer, he also was unable to stop a cordrazine-crazed Doctor Leonard McCoy from escaping the bridge.

His engineering skills were put to the test in four missions. When the Enterprise accidentally hurled back in time to the year 1969, he helped repair the engineering systems for the voyage back to 2267. On stardate 3468.1, he helped Commander Spock and Lieutenant Hikaru Sulu in breaking the Enterprise free from Apollo's force field. On stardate 3614.9, he tried to restore the engineering station after Redjac took over the ship's computer systems. He also helped repair the engineering systems on stardate 3715.3, when Lieutenant Commander Montgomery Scott tried to break the starship free from Vaal's hold.

Before he received his promotion, Leslie performed the duties of a helm officer on three missions. The first time occurred on stardate 1513.1, when the Enterprise orbited planet M-113. He took over the helm again on stardate 1704.2, after an infected Sulu abandoned his post. The third time happened on stardate 2124.5, when the ship discovered the rogue planet Gothos.

After his promotion, he operated the helm four more times. Most noteworthy assignment at the helm occurred on stardate 3088.7, when he performed some tactical duties. Under Kirk's orders, he fired the starship's phasers and destroyed Lazarus' ship.

At times, Leslie helped Doctor McCoy in sickbay as a medical assistant.

There is one occasion when Leslie performed the duties of a navigator.

Leslie served as a science officer on two different missions before he became a lieutenant In addition to his engineering accomplishments, Leslie was also well known for his skills as a security officer.

His final assignment occurred on stardate 5630.7, when Kirk discovered an assassination plot against Ambassador Kollos. He teamed up with Kirk, Spock and McCoy in response to the threat and ensured that no harm came to the Medusan ambassador.