[5] After six months in the traffic department, Mattinson began working as an in-betweener on Lady and the Tramp (1955), where he was mentored by Johnny Bond.

"[8] For One Hundred and One Dalmatians (1961), Mattinson provided in-between animation; he also drew illustrations for the Little Golden Books adaptation for the film.

During a storyboard meeting, the directors had criticized a sequence Mattinson had boarded—the introduction of Taran and Dallben—claiming that it wasn't ready to move forward into production.

[15] Having long desired to produce a film starring the "Fab Five" (Mickey, Minnie, Donald, Goofy, Pluto), Mattinson remembered the 1974 Disneyland Storyteller album titled An Adaptation of Dickens' Christmas Carol Performed by the Walt Disney Players written by Alan Young (who also performed the voice for Scrooge McDuck).

[16] In March 1981, at his wife's urging, Mattinson sent a two-paragraph pitch letter and the LP record to then-Disney CEO Ron Miller.

[14] According to Mattinson, the project was initially conceived as a "24-minute TV special to air annually" starting on Christmas 1982, but an industry-wide animators' strike that same year delayed production.

[1] During production, in 1984, Miller, who had been the film's producer, was forced out as CEO and president, and replaced by Michael Eisner and Frank Wells.

Using the assembled elements, an animatic was filmed and screened to Disney executives, but then-studio chairman Jeffrey Katzenberg declined to complete it.

[19][20] Throughout the 1990s, Mattinson worked as a storyboard artist on nearly every subsequent Disney theatrical animated film, including Beauty and the Beast (1991), Aladdin (1992), and The Lion King (1994).

Together, along with storyboard artist Vance Gerry, the trio were comically referred as the "Geriatricals", who would pitch adaptations of children's books and develop original story ideas.

[21] One of those discarded ideas Mattinson co-developed with Grant was Bitzy, which centered on an Indian elephant who leaves home to start a Hollywood career but ends up working in a used-car lot and falling in love.

[1] A year later, directors Stephen J. Anderson and Don Hall enlisted Mattinson to work as a story supervisor on Winnie the Pooh (2011), due to his earlier involvement with the 1960s featurettes.

[24] In March 2011, Mattinson announced that he was pitching an idea for a full-length animated feature with Mickey Mouse as the main protagonist.

[28][29] Mattinson also worked as a story artist for Wish (2023), which was also dedicated to his memory and marks the final film he was involved in prior to his death.

[30][31] On June 4, 2023, the Walt Disney Company presented a sculpture, featuring Winnie the Pooh characters, to Mattinson's family on the anniversary of his seventh decade-long career.