Together with his writing partner Barry Levinson, he formed a stand-up comedy act and wrote comedy (along with Rudy DeLuca) for shows like "The Tim Conway Show" (CBS, 1970) and "The John Byner Comedy Hour" (CBS, 1972). In 1973, Nelson left show business, moving with his wife and children to Mount Shasta in Northern California where he worked as a janitor, teacher, surveyor and carpenter before returning to Los Angeles four and a half years later. His experiences as a "drop-out" would lead him to produce a series of 52 documentaries syndicated under the title "America Still", which explored why artists choose a rural lifestyle. He resumed his career, making his feature debut in "...And Justice for All" (1979), scripted by his old friend Levinson, and has not slowed down since.

Nelson first came to prominence as the father in the movie "Poltergeist" (1982), a role he would reprise in the sequel "Poltergeist II: The Other Side" (1986). His high school football coach role in "All the Right Moves" (1983), starring Tom Cruise, served as a trial run for the part with which most people associate him, Hayden Fox, the hard-boiled but good-humored tough guy head coach of the Minnesota State Screaming Eagles, on the popular long-running sitcom "Coach" (ABC, 1989-97). Emmy-nominated three times for Outstanding Lead Actor in a Comedy Series for his work on the sitcom, Nelson won the award for the 1991-92 season and served as its executive producer from 1992. Prior to "Coach", he had played a district attorney on the short-lived "Chicago Story" (NBC, 1984) and won critical praise as US Air Force Colonel Raynor Sarnac in "Call to Glory" (ABC, 1984-85). Nelson added director to his credits with the last episode of "Call to Glory" and also helmed numerous episodes of "Coach". He adapted a Harry Grant story to the screen for his starring turn in the ABC movie "Ride with the Wind" (1994), which he also executive produced.

Though Nelson has worked extensively on TV, his collaboration with director Brian Gibson ("Poltergeist II") yielded three notable gems. He appeared in the critically-acclaimed HBO movie "Murderers Among Us: The Simon Wiesenthal Story" (1989), played a drug enforcement agent in the Emmy-winning miniseries "Drug Wars: The Camarena Story" (NBC, 1990) and portrayed gossip columnist Walter Winchell in "The Josephine Baker Story" (HBO, 1991). He also starred in the CBS sci-fi miniseries "The Fire Next Time" (1993) and acted the part of Kirk Douglas's son in "Take Mme Home Again" (NBC, 1994) among his many small screen performances. Equally prolific in features, Nelson played opposite Shelley Long in "Troop Beverly Hills" (1989), created the role of Chief Hyde in "Turner and Hooch", was a menacing drug dealer in "I'm Not Rappaport" (1996) and a district attorney in "Ghosts of Mississippi" (1996). Following his portrayal of a Donald Trump-like real estate developer in "The Devil's Advocate" (1997), he was reunited with Barry Levinson in the David Mamet-scripted "Wag the Dog" (1997), co-starring with Robert De Niro and Dustin Hoffman. He then appeared as Paul Walker's father in the secret society thriller "The Skulls" (2000).

Nelson next found continued success on the small screen with the horror telepic "Creature" (1998) and the miniseries "To Serve and Protect" (1999), playing one of three generations of Dallas police officers join together to track down the deadliest serial killer to ever terrorize Texas. After a stint in the pilot film for the bounty hunter series "The Huntress" (2000), Nelson co-starred to great effect in the inspired-by-true-events telepic "Dirty Pictures" (2000), playing an inflamed county sheriff intent on taking down a Cincinnati museum director (James Woods) after a controversial art exhibit by photographer Robert Mapplethorpe. The actor then returned to series television as the star of the crime drama "The District" (CBS, 2000-2004) playing no-nonsense Washington D.C. police commissioner Jack Mannion, a lounge singing former transit cop dedicated to cutting the district's incredibly high crime rate in half. Although never a critical darling, the series developed a loyal following and was a staple of CBS's crime show-centric schedule for four seasons.

Heading back to the big screen, Nelson took on a role that would likely become iconic, although it was a voice-acting gig only: As the voice of the reluctantly retired, slightly past-his-prime superhero Mr. Incredible in Disney/Pixar's "The Incredibles" (2004), Nelson seamless merged his own trademark persona with that of the befuddled ex-hero, creating a comic gem of a character that would likely delight audiences for generations. After the success of "The Incredibles," Nelson was part of the ensemble cast in "The Family Stone" (2005), playing the patriarch of a bohemian New England family.