His introduction of “intelligent design” came in his book ”Darwin on Trial”, which since Johnson presented evidence in the form of a mock trial (with legal standards of admissibility of evidence), rejected all scientific evidence in favor of anecdotal evidence – in addition to being (of course) utterly selective in what evidence to present. The point was, essentially, that since the evidence for theory of evolution didn’t provide absolute, logical, irrefutable proof, the theory has to be rejected (no one ever sees that kind of argument from misunderstanding of the role and standards of evidence pop up among climate change denialists, no?). The fact that intelligent design has failed utterly as a scientific theory does not seem to bother him.
Johnson’s vision of the mission of the Intelligent Design PR movement is not limited to evolutionary biology. Rather, the point is that all science lacks a proper theistic basis. Hence every field of science and indeed all public policy should be held hostage to theocratic organization. This is apparently why Johnson calls evolution the 'thin edge of the wedge' with which to 'split the log of materialism open'. This is a good resource on Johnson and his strategy. The idea is not to establish ID through science, but through public policy – hence the Discovery Institute’s focus, not on developing ID, but to get it into school curricula. See also this.
Thus Johnson is known for accepting not only creationism, but the whole full range of woo and crank ideas. He is, for instance, a HIV-denialist as well, having written several articles denying the link between HIV and AIDS.
Johnson has, however, remained relatively quiet the last 10 years after suffering a series of strokes, but he does make the occasional reappearance.
For fun, you can try scoring him on the crackpot index. Ed Brayton provides a brilliant guide here.
This is an interesting take on the whole creationist movement.
Diagnosis: Hyper-crackpot and one of the central founders of the denialist movement. His impact has been huge, but he seems to be semi-retired at present. Still dangerous, however.