Thorleifur spent time on loan at Augnablik during the 2019 and 2020 seasons, where he made 21 appearances and scored 10 goals in the 3. deild karla, the 4th tier of Icelandic football.
Thorleifur made 3 first team appearances for his boyhood club Breiðablik, 2 in the League Cup and 1 in the Úrvalsdeild before leaving for Duke.
[3] Following his second year at Duke, it was announced that Thorleifur had signed a Generation Adidas contract with Major League Soccer ahead of the 2022 MLS SuperDraft.
[5] He made his Dynamo debut on 27 February 2022, coming off the bench in the 81st minute during a 0–0 draw against Real Salt Lake in Houston's opening match of the season.
[9][10][11] Thorleifur was born in Reykjavík, Iceland to Úlfar Helgason and Gudfinna Kristofersdottir and was raised in the neighboring town of Kópavogur.