In 2016, Ifans played Fool alongside Glenda Jackson in Deborah Warner's production of King Lear, at The Old Vic.

Ifans was given the role of Atticus Finch in Aaron Sorkin's To Kill A Mockingbird to begin at London's Gielgud Theatre on 21 May 2020.

[13] Ifans appeared in many Welsh-language television programmes before embarking on his film career, including the comedy show Pobol y Chyff [cy], as well as performing at the National Theatre, London and the Royal Exchange, Manchester.

In 2004, his performance as Peter Cook in the TV film Not Only But Always won him the BAFTA Best Actor at the 2005 British Academy Television Awards.

Show Me Magic" and "Hometown Unicorn" by Super Furry Animals, "Mulder and Scully" by Catatonia, and "Mama Told Me Not to Come" by Tom Jones with Stereophonics.

In 2008, he appeared in "Six Days One June", one of three episodes of the TV series The Last Word Monologues, written by Hugo Blick and broadcast on BBC Two.

Other film roles include: Eyeball Paul in Kevin & Perry Go Large (2000), Nigel in The Replacements (2000), Iki in The 51st State (2001), William Dobbin in Vanity Fair (2004), and Vladis Grutas in Hannibal Rising (2007).

In the same interview, he announced that he would play the title role in the film Mr. Nice, based on the life of the drug smuggler Howard Marks.

It is a surrealistic biopic, which recreates the life of Welsh poet Dylan Thomas through flashbacks during the famous drinking binge at the White Horse Tavern in New York City which ended fatally during the fall of 1953.

It stars Ifans as Thomas alongside John Malkovich, Rodrigo Santoro, Romola Garai, Zosia Mamet, and Tony Hale.

Later it was revealed that Ifans was not actually present on set and only lent his voice for the character created through digital means, with his human form at the end of the film being from archival footage from The Amazing Spider-Man.