(covers information from several alternate timelines)
William McKenzie was a Human Starfleet crewman who served on the USS Voyager while the ship was stranded in the Delta Quadrant.
McKenzie in 2371
McKenzie worked as aide to Vice Admiral Patterson during his time aboard the ship when it still was in drydock. Together with Patterson he welcomed Captain Kathryn Janeway when she was beamed aboard and accompanied the two on their tour to the bridge and engineering. He was also with Patterson and Janeway when Seven of Nine hunted Captain Braxton through the corridors of Voyager. (VOY: "Relativity")
That same year, when Voyager had been in the Delta Quadrant for only a very short time, Captain Janeway watched him gazing out of a window in a corridor. (VOY: "The Cloud")
McKenzie was in the mess hall when Kes brought the Vhnori Ptera to the room and had a conversation with her. (VOY: "Emanations")
Later that year, he beamed down to Sikaris for shore leave and visited a marketplace with the rest of the crew. (VOY: "Prime Factors")
He was on duty in main engineering when the photonic lifeform broke out of the sample container and left the ship. (VOY: "Heroes and Demons")
McKenzie left the turbolift and entered the bridge when Captain Janeway tried to contact sickbay and activated The Doctor. (VOY: "Jetrel")
He was enjoying a meal in the mess hall when Neelix and Tom Paris started a food fight in 2372. (VOY: "Parturition")
He was working in engineering when Tom Paris broke the transwarp threshold in 2372. (VOY: "Threshold")
He was sitting at the table in the mess hall together with other crewmembers when Quinn suddenly appeared along with Janeway. (VOY: "Death Wish")
McKenzie was on duty in main engineering when a console exploded and three crewmembers, including Michael Jonas, were wounded. (VOY: "Investigations")
He was working in engineering when a group of Enaran scientists came aboard Voyager and later passed Chakotay and B'Elanna Torres in a corridor. McKenzie also attended the banquet, held for the Enarans in the decorated mess hall. Shortly after the banquet he witnessed the collapse of B'Elanna Torres in engineering and later attended the final ceremony for the Enarans in the mess hall when B'Elanna accused Jor Brel and explained the crimes of the Enarans. (VOY: "Remember")
McKenzie celebrated along Russell and a female officer the successful mission of Voyager and the return to the 24th century in the mess hall. (VOY: "Future's End, Part II")
When Captain Janeway hallucinated her own death and funeral in 2373, Crewman McKenzie blew the boatswain's whistle. (VOY: "Coda")
McKenzie stood in for Tuvok at the tactical station in 2373, while Tuvok was on an away mission aboard a Borg ship. (VOY: "Unity")
He was sitting at the table in the mess hall talking with Brooks. (VOY: "Favorite Son")
He visited the Paxau Resort when Kes experienced one of her timeshifts and witnessed the Krenim attack on Voyager. Later, in the normal timeline, he attended the party for Kes at the Paxau Resort holoprogram. (VOY: "Before and After")
McKenzie and Brooks transported equipment through a corridor when the ship prepared to enter Borg space in 2373. He was also working in main engineering at this time and when the crew made First contact with Species 8472. (VOY: "Scorpion")
He was working on an aft station of the bridge when the Voyager formed the alliance with the Borg and when the ship was brought into fluidic space by Seven of Nine. (VOY: "Scorpion, Part II")
McKenzie assisting a wounded crewmember
McKenzie passed Tuvok and Captain Janeway in a corridor when the two were on their way to the brig where Seven of Nine was held. (VOY: "The Gift")
McKenzie was on duty in engineering during the test of the transwarp drive. He was also among the engineers who had to be evacuated from engineering when the warp core was ejected. (VOY: "Day of Honor")
He was working in cargo bay 2 as part of the team which was tasked to investigate Seven of Nine's irrational behavior. (VOY: "The Raven")
McKenzie was on duty in engineering when Tuvok surprised the kissing couple Tom Paris and B'Elanna Torres and when Seven of Nine uncovered the Srivani scientist Alzen. He also brought a suffering crewmember into sickbay. (VOY: "Scientific Method")
In an alternate timeline created by the Krenim weapon ship while erasing species and civilizations, McKenzie was wounded and cared for in the triage mess hall. Later, he was also among the crewmembers in the mess hall who listened to Captain Janeway's speech before he left the Voyager aboard an escape pod. (VOY: "Year of Hell")
In 2375, he was at the celebration that took place in Voyager's engineering area to introduce the modified warp core by the quantum slipstream drive. (VOY: "Timeless")
McKenzie, Timothy Lang and Renlay Sharr attended at the reception in the mess hall, given to the Ambassador Tomin. (VOY: "Someone to Watch Over Me")
McKenzie was part of Neelix's story he told the Borg children during the power shutdown of Voyager. In this story, he was among the crewmembers who gathered in engineering. (VOY: "The Haunting of Deck Twelve")
He, Brooks, and a command division officer, between other crewmembers, were in a meeting at astrometrics to watch the positions of the racers. (VOY: "Drive")
In 2377, he was brainwashed by the Quarren after they irradiated Voyager and forced the evacuation of its crew. He appeared descending the Central Power Facility stairs when, his identity was reassigned to that of a laborer working along with most of the rest of the crew for approximately three weeks in the largest city on Quarra, when their memories were altered and they were unaware that nearly the entire Voyager crew was abducted by Dr. Kadan. (VOY: "Workforce"). McKenzie's personnel file at the plant was one of many Voyager crew files accessed by Tuvok, whose memory was still partially intact. (VOY: "Workforce, Part II")
He was in the mess hall when Q distributed food to the crewmembers. (VOY: "Q2")
McKenzie was in the mess hall when Neelix distributed minutes for Voyager crewmembers to communicate with the Alpha Quadrant. (VOY: "Author, Author")
He celebrated First Contact Day with the rest of the crew in 2378 in the mess hall aboard the Voyager, where he had a conversation with Tuvok. Later, he stood in a row to bid farewell to Neelix, who left the ship. (VOY: "Homestead")