Marcus has established himself as one of the UK's foremost auteur documentary directors. His directorial debut in 2015, Muslim Drag Queens for Channel 4 (UK), narrated by Sir Ian McKellen, spearheaded the network's True Colours alternative voices initiative in 2016 and earned Marcus a nomination for Breakthrough BAFTA talent.

Since then, World's Collide: The Manchester Bombing, examining the terror attack at an Ariana Grande concert in his home city of Manchester (UK), garnered a 2023 BAFTA nomination for Best Factual Series. In 2021, Marcus' first feature documentary for ITV (UK), In Cold Blood, investigating the largest treatment disaster in NHS history, won an International Emmy for Best Current Affairs film. It also received UK nominations for Best Current Affairs film at the Royal Television, Grierson, and Amnesty International Awards. The Telegraph described it as "a staggering investigation to make your blood boil".

Marcus gained earlier recognition in the United States with his 2017 BBC series American High School, depicting life inside a 99% African American high school in South Carolina. The series sold internationally and enjoyed a six-week run on National Geographic.

2019 saw Marcus win a Grierson award for his Channel 4 (UK) film, The Football Club. That same year, he received another BAFTA nomination (Best Single Documentary) for The Murder of Jill Dando, the BBC's highest-rated documentary of the year. In 2020, he filmed and directed Football, Prince William and Our Mental Health with the Duke of Cambridge.

Marcus directed the Netflix series Who Killed Jill Dando in 2023. In August 2024 he completed a two-part Peacock Special for NBC Universal, The Girl on The Milk Carton and is now serving as Director and Showrunner of the forthcoming mini series, Dirty Rotten Scandals: The Dr Phil Show for Versant Media.