Schnellenberger worked extensively as an assistant coach at the college and professional levels, including as part of the staff of the undefeated 1972 Miami Dolphins.

Schnellenberger left Alabama in early 1966 to take a job in the National Football League (NFL) as offensive ends coach of the Los Angeles Rams under George Allen.

[12] Initially named as wide receivers coach,[12] Schnellenberger was the Dolphins' offensive coordinator by July.

[14] Schnellenberger signed a three-year contract to succeed John Sandusky as head coach of the Baltimore Colts on February 14, 1973.

Drawing from the boot camp methodology learned from mentors Bryant and Shula and a pro-style pass-oriented playbook not yet the norm in college football, Schnellenberger introduced a passing game at Miami that gave them advantage over teams not equipped to defend such an aggressive offensive passing attack.

His eye for talent in this area led to many programs around the nation paying greater attention to South Florida high school prospects.

Following the season, Schnellenberger resigned to become part-owner, president, general manager and head coach of The Spirit of Miami of the United States Football League, a relocated Washington Federals franchise.

Nonetheless, at his opening press conference, he stunned reporters and fans by proclaiming the program "is on a collision course with the national championship.

They won them both, including a 34–7 thrashing of the Alabama Crimson Tide in the 1991 Fiesta Bowl, capping a 10–1–1 season and the school's then-highest appearance in the final AP poll (14th).

He recalled in a 2012 interview that his departure was a direct result of the impending creation of Conference USA:[28]I didn't leave because of money.

[29] Repeating his bluster upon taking the Louisville job, Schnellenberger declared, "They'll write books and make movies about my time here."

On December 19, 1995, Schnellenberger resigned unexpectedly after one season, stating that "in recent months a climate has developed toward the program, understandably in some cases and perhaps unfairly in others, that has changed my outlook on the situation.

"[31] Schnellenberger has not been held in high regard by Sooner fans, in part because he made no secret of his lack of interest in Oklahoma's football history (his comments after the 1994 Copper Bowl notwithstanding).

On his statewide tour, he vowed to put together a team that would make "Sooner Nation" forget about head coaches Bud Wilkinson and Barry Switzer.

After leaving Oklahoma, Schnellenberger decided to try the financial world and became a bond salesman, passing the certification exam on his third try.

In 2008, Schnellenberger led his 6–6 FAU Owls to a post-season bid at the Motor City Bowl against the Central Michigan Chippewas.

[39] Their oldest son Stephen was diagnosed as an infant with a rare form of endocrine cancer but lived a normal childhood and became an insurance broker in Florida; however, during a 2003 surgery, his heart stopped and he suffered brain damage that left him in a semi-comatose state.

[41][42][43] The home was described as "like a museum" in a 2017 profile for The New York Times due to displayed items from such notable figures as George H. W. Bush, Burt Reynolds, and Joe Namath.

[44] Schnellenberger was known for his gravelly baritone voice and was known for smoking a trademark pipe, but he gave it up after he found out his son was diagnosed with cancer.

[24][25] During his time at Miami and Louisville, Schnellenberger was known for wearing a distinctive suede jacket and a conservative striped tie, echoing the dress of his mentors such as Bear Bryant, though he more often wore golf shirts as coach of FAU.

[25] He was also famous for his colorful press conference quips, so much so that the Louisville Eccentric Observer ran a feature called "SchnellSpeak of the Week".

[45] Schnellenberger played the referee in the football game scene in the Robert Altman-directed 1970 war film M*A*S*H. He said of his cinematic experience, "That was hard work.

"[46] After his retirement, Schnellenberger was retained by Florida Atlantic University (FAU) and named the first ever "Ambassador at large" his main responsibility was to help drive fundraising efforts for the athletic department.

[48][49][50] In 2023, Louisville and Miami jointly announced that the schools would compete for the new Schellenberger Trophy for future rivalry matchups.

The Louisville Sports Commission unveiled the trophy, which is a bronzed pair of Schnellenberger's famous cowboy boots.