[4] In 2019, Gallagher began portraying Number Five on the Netflix superhero series The Umbrella Academy, which is his breakthrough role and has brought him widespread recognition and critical acclaim.

[5] In 2018, Gallagher was appointed as a UN Environment Goodwill Ambassador for North America, becoming one of the youngest people to hold the position.

[15][16] His portrayal won him critical praise; as The New York Times' reviewer put it, he "carries the show as far as he can".

[17] Daniel Fienberg of The Hollywood Reporter wrote that "Gallagher is very good in the tricky part of a fifty-something-year-old man trapped in the body of a schoolboy".

[6][7][8] In the year 2021, he gave an interview for the fashion magazine Vanity Teen, reviewing his career and his social influence on youth.