He has written songs for artists such as Barbra Streisand, Idina Menzel, Andrea Bocelli, Lauren Daigle, Nicole Scherzinger, and others, and has often produced the works.

Growing up, his parents shared their favourite music with him including Stevie Wonder, The Beatles,[4] Donny Hathaway records, which he has cited as a major influence on his work.

[11] Myrin wrote the song "Period" as the opening theme of the Japanese anime series, Fullmetal Alchemist: Brotherhood.

On New Year's Eve 2014, he performed the song for hundreds of thousands of people live at The Brandenburg Gate on a bill with Pet Shop Boys, Bonnie Tyler and Loreen.

[24] The song took off and Walker-Smith and Gabriella Rocha collaborated for a Portuguese version called "Teu Espírito", and it was released in Brazil.

[26] Later in the year, Myrin was chosen by Universal Music Publishing's stable of songwriters to travel to Rio de Janeiro, Brazil, to compose with Vanessa da Mata Ana Carolina, and Antonio Villeroy.

"Being able to absorb these different genres is fascinating," Myrin told UBC – União Brasileira de Compositores magazine while he was in Brazil.

He co-wrote and co-produced four songs on Streisand's album Walls including "What's On Your Mind", "Don't Lie to Me",[28] "The Rain Will Fall",[29] and "Better Angels".

[32] Myrin worked with France's Gaëtan Roussel on his Traffic album, including songs "Tu Me Manques", "Le jour et a nuit" – on which he played piano, and the single, "Hope" featuring his vocals.

[34][35] In 2019, Myrin performed "That's What Friends Are For", together with his songwriting collaborator Carole Bayer Sager, as well as Patti LaBelle, Desmond Child and Jordan Smith, at the Songwriters Hall of Fame 50th Annual Induction and Awards Dinner in New York City, in honour of Carole Bayer Sager receiving the Johnny Mercer Award.

[36] Later that year, in sessions at Myrin's Hollywood studio, he co-wrote "Thank Me Later", "Lonely" and Stranger" with Devon Gilfillian for his debut album Black Hole Rainbow.

[39] Together with Pussycat Doll, Nicole Scherzinger, Myrin co-wrote "Victorious", in March 2019, which they performed at the Special Olympics World Games in Abu Dhabi that year.

[41] In June 2019, Celine Dion recorded "Play Me Like A Love Song", which Myrin wrote with Carole Bayer Sager.

[4] The lyrics of "Not Alone", co-written with Crispin Hunt (Lana Del Rey, Florence and the Machine, Ellie Goulding), were inspired by a time when Myrin lived in London and found himself overcome by loneliness in the big city.

"Not Alone" features string arrangements by Erik Arvinder (Childish Gambino, John Legend) conducting the Stockholm Symphony,[49] and was co-produced by Elias Kapari.

[50] The music video for "Not Alone" was filmed in Joshua Tree featuring Myrin driving solo across the Mojave desert from sunrise to sunset, a visual of his admitted soul searching journey that year.

In 2017, he wrote and sang the song, "Sadness (Who's Laughing Now)" for Sony Pictures' film, Rough Night, featuring Scarlett Johansson.

"[60] In 2020, he was commissioned to write the theme song, "Just a Breath Away (Noel)", for the holiday film, The Three Wise Men, directed by Yarrow, and featuring narration from the late Andy Griffith.