[11] In 2002, he made his Broadway debut in the stage version of The Graduate, succeeding Jason Biggs, opposite Kathleen Turner and Alicia Silverstone.

In 2004, Radnor starred in The Paris Letter alongside his future How I Met Your Mother co-star Neil Patrick Harris.

[13] In July 2008, he starred opposite Jennifer Westfeldt in the premiere of the play Finks,[14] written by Joe Gilford and directed by Charlie Stratton for New York Stage and Film.

Radnor and Lee had known each other for "twelve or thirteen years after meeting on the set of How I Met Your Mother" eventually writing songs together.

[21] Their debut album, Radnor & Lee was released on November 10, 2017,[22] "receiving widespread praise across the board, and cementing the duo as an accomplished pair of artists," stated Rolling Stone.

[23] On February 19, 2020, Spin announced Radnor and Lee's sophomore album, Golden State, out on Flower Moon Records, and premiered the first single "Outside In.

Rolling Stone premiered the first single, "The High Road," March 10, 2021, calling it a "gently lilting ballad about the breakdown of a friendship on which he sings... which features deep textures of piano, acoustic guitar, organ, and snapping fingers.

"[26] The EP's 2nd single, "You Feel New," premiered on March 31, 2021, with features in Rolling Stone, NPR, American Songwriter and Paste Magazine.

[27] From 2020 to 2023 he starred as Lonny Flash Amazon Prime series, Hunters acting alongside Logan Lerman and Al Pacino.