The Mutts characters appear on the New Jersey Animal Friendly license plates, which fund state pet population programs.

[5] His first high-profile ongoing freelance work was providing illustrations for the Russell Baker Observer column for The New York Times Sunday Magazine from 1978 to 1993.

In 2005, McDonnell curated an exhibition — "Top Dogs: Comic Canines Before and After Snoopy" — at the Charles M. Schulz Museum in Santa Rosa, California.

[12][1] In 2006, the Mutts characters were featured in the United States Marines' Toys for Tots holiday poster, and in 2007 were part of the American Library Association's "Read!"

The 2007-2008 Mazdaspeed Team featured Mutts-themed vehicles promoting pet adoption and the work of the Humane Society of the United States.

[1] In 2009 McDonnell collaborated with author Eckhart Tolle to create Guardians of Being, a philosophical book about nature and the present moment.

[17] McDonnell is a member of the board of directors of the Humane Society of the United States, the Fund for Animals,[1] and The Charles M. Schulz Museum.

[18] McDonnell and his wife Karen O'Connell — whom he met in the late 1970s while both members of the punk band Steel Tips — reside in Princeton, New Jersey,[1] with their dog Amelie, and their cat, Willie Lebowsky.

Their Jack Russell Terrier, Earl, who was the inspiration and constant muse for the Mutts character of the same name, died in November 2007 after living with McDonnell for over 18 years.