Mackenzie was born in the Stirling area of Scotland and grew up in Ashby-de-la-Zouch, Leicestershire, England. Racing runs deep in his family: his father Niall was a three-time British Superbike champion, and his brother Taylor also competed professionally before retiring in late 2021. Tarran began racing in the Aprilia Superteen Rookies Cup and progressed through the junior classes.
In 2011, Mackenzie competed in the British 125cc Championship. The following year he was selected for the prestigious Red Bull MotoGP Rookies Cup, a series aimed at identifying future Grand Prix talent and run across various European circuits. He claimed the British Supersport 600 Championship title in 2016, the same year his brother Taylor won the British Superstock 1000cc Championship — a remarkable parallel achievement for the Mackenzie family.
In 2017, Mackenzie made his Moto2 world championship debut as a replacement for Danny Kent, scoring one championship point with a best finish of 15th and finishing 37th overall in the standings. The short campaign nonetheless demonstrated his ability to adapt to world-level machinery.
Mackenzie became a regular in the British Superbike Championship from the 2018 season onward. He built steadily toward the front of the field, and in 2021 he claimed the BSB title at the final round in October. The championship was a vindication of years of development work and placed him in line for opportunities at world level.
During 2022, Mackenzie faced a disrupted season. He missed the opening rounds at Silverstone and Oulton Park due to injury, returning at Donington Park partially recovered. New physical complications — arm pump caused by compensating for an inability to use normal leg pressure on the footrests — further hampered his form. A crash at Oulton Park on 25 September ended his season when he broke his left femur after sliding under an oncoming motorcycle.
Mackenzie received a factory Yamaha wildcard offer for the 2022 World Superbike Championship, targeting up to three events. A leg injury ruled him out of the scheduled Assen round. At Donington Park in July, he started three races: a DNF in the Superpole sprint after contact with Xavi Vierge resulted in a five-place grid penalty for the final race, where he placed 15th.
For 2023, Mackenzie moved to the World Supersport Championship with the MIE Honda team run by Midori Moriwaki. He claimed his first world championship race victory in a wet-weather event where he elected to stay on dry tyres as rivals lost time in the pits for wheel changes — a calculated gamble that paid off.
In 2024, he stepped up from World Supersport to World Superbikes, remaining with MIE Honda alongside teammate Adam Norrodin.
Mackenzie's career represents the continuation of a Scottish motorcycle racing lineage that spans three generations of competition. His 2021 BSB title confirmed his status as one of the leading British road racers of his generation, and his progression to the World Superbike Championship fulfilled the ambitions his father's era had set as a benchmark for Scottish riders. His willingness to accept risk in strategic race decisions — illustrated by the dry-tyre call in his world-level debut victory — is regarded as a hallmark of his competitive style.