In his junior year in 2015, Tiernan claimed the runner-up spot, closely contesting the title with University of Oregon’s Edward Cheserek.

Cheserek entered the 2016 NCAA Division Cross Country Championships as the clear favorite, having won the competition an unprecedented three years in a row.

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.

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.

[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.

[19] 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.

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.

[23][24] Nonetheless, Tiernan persevered in finishing the race, albeit in last place, and subsequently required a wheelchair and medical care.