Initially an actor after graduating college, he became a screenwriter, notably creating Rescue Me and adaptations of novels, plays, and memoirs.

He attended Barton Peveril College at Eastleigh, Hampshire, taking A-levels in Drama, English Literature, Physics and Biology.

Nicholls says that a turning point in his career came when a friend gave him a copy of P. J. Kavanagh’s memoir The Perfect Stranger, which tells the author's own tale of maturation, finding love, and discovering his path in life.

[4] Nicholls's third novel, One Day (2009), became an international bestseller and has sold over six million copies worldwide, being translated into 40 languages.

He wrote a screen adaptation of his novel, One Day, which was made into a film starring Anne Hathaway and Jim Sturgess.

The play, starring James Nesbitt, Saffron Burrows, Catherine Tate and Gael García Bernal was just 10 minutes long.

He wrote Patrick Melrose (2018), a five-part television series based on Edward St Aubyn's novels, and received a nomination for the Primetime Emmy Award for Outstanding Writing for a Limited Series, Movie, or Dramatic Special for his work on the show.

Nicholls lives in Highbury in north London, with his partner of more than 25 years, Hannah Weaver, a script editor.