Col. Radcliffe Thornberry, R.A.F., (Ret.) is the father of Nigel. Listening to him, it is easy to hear where his son Nigel got his trademark voice, as well as his sense of adventure. Radcliffe lives along with his wife Cordelia at Thornberry Manor in Scotland. A former member of the Royal Air Force, he enjoys hang gliding, playing the bagpipes, and loves a good adventure.
Many of Nigel's more admirable qualities can be traced back to his father, who helped his son nurse sick animals back to health when Nigel was just a little boy. Although he and Cordelia seem very different, he loves his wife tremendously but isn't afraid to go on a perilous adventure against her wishes. Both he and Nigel are voiced Tim Curry, which explains why Nigel's and Radcliffe's voices are the same.
Radcliffe Thornberry is the paternal grandfather of Eliza and Debbie, and the adoptive paternal grandfather of Donnie. He is a recurring character in the series and appears in several episodes. His voice is similar to that of his son.
Radcliffe has peach skin and white eyes with black pupils. He is bald but does have a mustache. He is normally shown wearing a tan jumper, a sky-blue shirt, a dark blue necktie, blue pants, and brown shoes.
Radcliffe appears in several episodes of The Wild Thornberrys. His first appearance was in "Sir Nigel", though he was mentioned before this episode aired in "Have Yourself a Thornberry Little Christmas". Radcliffe and Cordelia both appear in The Wild Thornberrys Movie.