[8][9] He grew up in what he calls a "cosmopolitan house", as they all communicated in English and his father, an Arya Samaji, would begin his day by reciting Hindu, Sikh and Christian prayers.

This led the then headmaster Gulab Ramchandani, who was Hiroo's uncle, to write a very "emotional letter" to her, declining admission for her son.

Johar started his career in the entertainment industry as an actor, when he played the role of Shrikant in the 1989 Doordarshan serial Indradhanush.

As a child, he was influenced by commercial Indian cinema: He cites Raj Kapoor, Yash Chopra and Sooraj Barjatya as his inspirations.

[21] Johar was one of the personalities who was targeted by the late actor's fans for allegedly encouraging nepotism by casting children of established Bollywood stars in his films instead of outsider talents.

The first half of the film centers on a college love triangle between an insensitive boy (Shah Rukh Khan), his tomboyish best friend (Kajol), and the prettiest girl at the college (Rani Mukerji), while the second half centers on the now-widowed boy's attempt to reconnect with his best friend who is now engaged to marry a businessman (Salman Khan).

Johar's third directorial venture was the ensemble musical romantic drama Kabhi Alvida Naa Kehna (2006), which dealt with the controversial subject of marital infidelity and dysfunctional relationships set against the backdrop of non-resident Indians (NRIs) living in New York City.

"[32] The script of the film which was co-written by Johar received recognition by a number of critics and was invited to be included in the library of the Academy of Motion Picture Arts and Sciences.

The plot follows a Muslim man with Asperger's syndrome and his Hindu wife, played by Shah Rukh Khan and Kajol, who live in San Francisco and face racial prejudice after 11 September attacks.

"[35] Johar then teamed up with Zoya Akhtar, Anurag Kashyap, and Dibakar Banerjee for Bombay Talkies (2013), an anthology film released to celebrate the centenary year of Hindi cinema.

The plot of Johar's film followed a magazine editor (Rani Mukerji) who discovers that her husband (Randeep Hooda) is gay after an interaction with an intern at her office (Saqib Saleem).

Johar teamed up again with Zoya Akhtar, Anurag Kashyap, and Dibakar Banerjee for Lust Stories (2018), an anthology film released on Netflix that served as a follow-up to Bombay Talkies.

Johar's segment revolved around a newly married schoolteacher (Kiara Advani) whose husband (Vicky Kaushal) fails to recognize her lack of sexual satisfaction.

[38] With a screenplay by Sumit Roy, dialogues by Hussain Haidry, and music composed by A. R. Rahman, it was supposed to be the second film directed by Johar that was not written by himself.

His segment in the film told the story of a newly-married woman (Mrunal Thakur) and her experience with her husband's (Avinash Tiwary) post-traumatic stress disorder.

In July 2021, it was announced that Johar would direct a romantic comedy instead, titled Rocky Aur Rani Kii Prem Kahaani.

Many of these films have become major critical and commercial successes including Kal Ho Naa Ho (2003), Dostana (2008), Wake Up Sid (2009), I Hate Luv Storys (2010), Agneepath (2012), Yeh Jawaani Hai Deewani (2013), 2 States (2014), Humpty Sharma Ki Dulhania (2014), Kapoor & Sons (2016), Dear Zindagi (2016), Badrinath Ki Dulhania (2017), Raazi (2018), Simmba (2018), Kesari (2019), Good Newwz (2019), Sooryavanshi (2021) and Brahmāstra: Part One – Shiva (2022).

In addition to working as an assistant director on Dilwale Dulhania Le Jayenge (1995), Johar was an actor in the film, playing the minor role of a friend of Shah Rukh Khan’s character.

He made his full-fledged acting debut alongside Ranbir Kapoor and Anushka Sharma in Anurag Kashyap’s period drama Bombay Velvet (2015), in which he played the main antagonist.

Critic Sarita A. Tanwar commented, "The only consolation in the film is Johar who brings a lot of dignity to the character of Khambatta... which is commendable since this is totally outside his comfort space.

Johar is the host of Koffee with Karan, a talk show where he interviews actors, directors, producers, and other prominent members of the Hindi film industry.

[47] In January 2022, he appeared as a judge on the Colors TV's show Hunarbaaz: Desh Ki Shaan along with Mithun Chakraborty and Parineeti Chopra.