Cross country

Tiernan competed for the Wildcats at Villanova University, where he earned All-American honors at each NCAA Division I Cross Country Championships in which he competed.^[7] As a freshman in 2013, Tiernan placed ninth, beating 244 other runners. He finished in eighteenth place the following year. In his junior year in 2015, Tiernan claimed the runner-up spot, closely contesting the title with University of Oregon’s Edward Cheserek.^[8] Tiernan led much of the race, before Cheserek pulled away in the final mile.

Cheserek entered the 2016 NCAA Division Cross Country Championships as the clear favorite, having won the competition an unprecedented three years in a row.^[9][10] The race took place LaVern Gibson Cross Country Course in Terre Haute, Indiana, and it quickly unfolded into a three-man race between Tiernan, Cheserek, and Syracuse junior Justyn Knight.

Tiernan broke away from the other competitors in the final stages of the race, becoming the first Villanova man to win the NCAA Championships since Victor Zwolak in 1963. Knight finished in second place, leaving Cheserek in an unexpected third. Following his victory, the U.S. Track & Field and Cross Country Coaches Association named Tiernan its National Athlete of the Year.^[11] In reflecting on the race, LetsRun.com wrote that Tiernan "defeated arguably the most dominant runner in collegiate history in a performance that will be talked about for decades to come."^[12]

Track

On the track, Tiernan recorded his highest finish at the 2016 NCAA Division I Outdoor Track & Field Championships, held in June, finishing third in 13:27.07.^[13] Following the NCAA Championships, Tiernan continued to compete in Europe and achieved a time of 13:20.88 in the 5000 m at the Paavo Nurmi Games in Turku, Finland, on June 29.^[14] This time was below the Olympic standard of 13:25.00. The following month, Australia confirmed his selection to the Olympic team for the 5000 m.^[15] At the 2016 Summer Olympics in Rio de Janeiro on August 17, Tiernan participated in the 5000 m race but finished thirteenth in his heat, which did not qualify him for the final.

2017-19

Tiernan revealed his decision to sign a professional sponsorship deal with Nike on 26 March 2017.^[16] The announcement came after he finished thirteenth at the 42nd IAAF World Cross Country Championships in Kampala, Uganda. The following May, he won the 10000 m of the Palo Alto Payton Jordan Invitational in a world-leading time and personal best of 27:29.81.^[17]

In August, Tiernan competed in the 2017 World Championships in Athletics. He placed last among twenty two competitors in the 10000 m. During the 5000 m, he took the lead at the 3000 m mark.^[18] However, he relinquished his lead shortly thereafter, eventually fading to eleventh place by the finish. Tiernan returned to compete at the 2019 World Championships in Athletics. In the 5000 m, he finished in tenth place during the heats, which was not enough to secure a position in the final. Following his participation in the world championships, Tiernan joined the Nike-sponsored Oregon Track Club.^[19]

2020-21

On 5 December 2020, Tiernan set a personal best of 27:22.55 in the 10,000m of The Track Meet in San Juan Capistrano, California.^[4] His time was an Australian record, surpassing the previous mark set by Stewart McSweyn. Tiernan's national record stood for 458 days before being surpassed by his countryman Jack Rayner on 8 March 2022.^[20]

After a hiatus due to the COVID-19 pandemic, Tiernan returned to competition in January 2021 at the Las Vegas Gold Half Marathon. He won the race in 1:02:38, covering the last five kilometers in 13:58.^[21] Three weeks later, he dropped down in distance to contest the mile at the Washington Invitational in Seattle, Washington, winning in 3:56.82.^[22]

The Australian competed in the 10,000m event at the 2020 Summer Olympics on 30 July 2021. During the race, he suffered from heat exhaustion, collapsing twice.^[23][24] Nonetheless, Tiernan persevered in finishing the race, albeit in last place, and subsequently required a wheelchair and medical care. Due to this episode of heat exhaustion, he withdrew from the subsequent 5000 m event at the Games, despite being entered to compete.

2022- present

In January 2022, Tiernan left the Nike Oregon Track Club to compete for PUMA.^[25] Tiernan placed sixth at the Aramco Houston Half Marathon in Houston, Texas, on 16 January, clocking a time of 1:00:55. This was the second-fastest time ever recorded by an Australian in the event, with only Brett Robinson having run faster.^[26] In October 2022, he made his marathon debut at the Chicago Marathon, finishing in sixteenth place with a time of 2:11:02.^[27]

Tiernan placed fourth at the 2024 Chevron Houston Marathon in 2:07:45 on the 14 January.^[28] His time was the second fastest marathon ever by an Australian and met the qualifying standard for the 2024 Summer Olympics.