Always wanting to write about George VI, Seidler started researching in the 1970s. After finding the surviving son of Lionel Logue, Dr. Valentine Logue, now a retired brain surgeon, he wrote him in 1981. In turn, Logue was keen to talk with Seidler and even share the notebooks his father kept while treating the King, but on the condition that he received "written permission from the Queen Mother" first. Upon writing to her, Seidler received a reply from her private secretary, asking him not to pursue the project during her lifetime. Consequently, Seidler abandoned the project in 1982.

The Queen Mother died in 2002, but Seidler didn't start the work until 2005, when he suffered from throat cancer, and returned to the story during a bout of creative work it inspired. Eventually he wrote the first draft of his screenplay, and his then-wife and writing partner suggested that he rewrite it as a stage play, as an exercise. She felt that the "physical confines of the stage would force him to focus on the key relationships in the story, without the distractions imposed by concern for cinematic technique." In 2011 Seidler won a BAFTA award for Best Original Screenplay, and later an Academy Award for Best Original Screenplay for the film The King's Speech.

In late 2005 Seidler was diagnosed with bladder cancer, but he is currently (2011) in remission.

Numerous forms of speech therapy failed him, until, at 16, he had a breakthrough. "I resolved that if I was going to stutter for the rest of my life, people were going to be stuck listening to me. I had been depressed, but now I was angry – I decided I deserved to be heard." That is when, in rage he spoke the 'F' word, or "naughty word" as he recalled decades later. Two weeks later he auditioned for his school play, Shaw's Androcles and the Lion and even got a small role, of a Christian eaten by a lion. In 2005, he used it in a scene in his stage play about George VI. Seidler later attended Cornell University, where he graduated with a A.B. in English in 1959.