Lilly began his career as a defensive end in 1961, but midway though the 1963 (his third) season Cowboys coach Tom Landry moved him to defensive tackle. Lilly made the adjustment becoming the main man in Dallas's vaunted "Doomsday Defense". As a tackle, Lilly was a first-team All-NFL choice every year from 1964 through 1969, then again in 1971. On January 17, 1971, the Cowboys finally made the Super Bowl, against the Baltimore Colts, only to lose the game, 16-13, on a field goal in the final nine seconds. However, in 1972 he and the rest of the Cowboys won the Super Bowl against the Miami Dolphins, 24-3. His 29-yard sack of Dolphin quarterbackBob Griese (an NFL Record) is one of the most memorable plays in Super Bowl defensive history. This was the signature play of his 14-year hall-of-fame career.
Lilly was named an All-Pro seven times, and was selected to play in 11 Pro Bowl games. He was also drafted in the second round (14th overall) in the 1961 AFL Draft by the Dallas Texans (now Kansas City Chiefs) prior to the merger with the NFL. His greatest assets were his pass-rushing skills and his ability to slice plays open with his agility and instincts. He had a distinct stance, the so-called four-point stance, placing both hands on the field instead of the more usual one, generating greater force when rushing straight ahead. Lilly's agility and quickness helped him score four defensive touchdowns in his career. His first was returning an interception 17 yards in 1964 while the other three came on fumble recoveries.
NFL Films did an eight-minute feature on Lilly and called him the "unblockable, unstoppable, force of The Doomsday Defense". Lilly was regularly double and triple teamed for the majority of his career because of his effectiveness as a defensive tackle. Lilly played in 196 consecutive regular-season games. The only NFL game he would miss in his career, was the 1973 NFC Championship Game loss (10-27), December 30, against the Minnesota Vikings, due to a leg injury. Lilly injured his hamstring in the Cowboys victory (22-10) against the Denver Broncos, December 2, 1973. In the first play of the 1973 NFC Divisional playoff game, three weeks later on December 23, versus the Los Angeles Rams (Cowboys 27-Rams 16) he re-injured this same hamstring.
Affectionately known as "Mr. Cowboy," his name was the first inscribed in the Dallas Cowboys Ring of Honor, above Texas Stadium and the current Cowboys Stadium. The Cowboys had a Bob Lilly Day on November 23, 1975, to honor him and make Lilly the first inductee into The Ring of Honor. He has attended every induction of each Ring of Honor inductee since.
Lilly was inducted into the Pro Football Hall of Fame in 1980, his first year of eligibility, and was the first player who spent his entire career with the Cowboys to be elected to the Pro Football Hall of Fame. He entered the Hall of Fame along with former teammate Herb Adderley (for two seasons), as well as David "Deacon" Jones and Jim Otto. The Sporting News named him a member of the All-Century NFL Team and "the greatest defensive tackle in NFL history". Lilly, Adderley, and Jones were all drafted in 1961. Tom Landry said of Lilly: "As I've said before, another Lilly won't come along in my time. We're observing a man who will become a legend". This comment is from the 1972 Street and Smith's Pro Football Yearbook. He also said that "Nobody is better than Lilly". He is a member of the National Football League 1960s All-Decade Team and National Football League 1970s All-Decade Team.
In 1999, he was ranked number 10 on The Sporting News' list of the 100 Greatest Football Players, the highest-ranking defensive lineman and the highest-ranking Cowboy. The only defensive players ranked ahead of Lilly were Dick Butkus and Lawrence Taylor. Sports Illustrated named him one of the ten most revolutionary defensive players.
Although the Cowboys don't have a practice to retire jersey numbers, Lilly is the only player to wear #74 in team history (with the exception of pre-season games).
When he began traveling with the Cowboys, Lilly regularly had his camera at his side. His interest in photography began when he was named to the College Football All-America Team in 1961, which was sponsored by Kodak. As a part of the honor he was given a 35-mm camera and a year's supply of film by the company. Before and after games, he spent an increasing amount of time studying and photographing old sports stadiums. Lilly co-authored the 1983 book Bob Lilly Reflections with sportswriter Sam Blair, featuring scores of his black-and-white photographs of teammates in candid poses. He was also a photographer for the Texas Air National Guard where he was an Airman 2nd Class. Lilly spent two weeks in 1967 as part of the Texas National Guard at the Rhein-Main Air Base in West Germany.
^Lilly, Bob, with Kristine Setting Clark (2008), A Cowboy's Life, Chicago, Illinois: Triumph Books, p. 3, ISBN978-1-60078-101-8 Monk, Cody (2004). Legends of the Dallas Cowboys. Sports Publishing LLC. p. 48. ISBN978-1-58261-707-7. Anderson, Dave (1967). Great defensive players of the NFL. Random House. p. 83. ISBN978-0-394-80197-1. Smith, Don (1988). NFL Pro Football Hall of Fame All Time Greats. Gallery Books. p. 147. ISBN978-0-8317-6300-8. National Football League (1992). Official 1992 National Football League Record & Fact Book. Workman Pub. Co. ISBN978-1-56305-247-7. Smith, Ron (1997). NFL Football: The Official Fan's Guide. Triumph Books. ISBN978-1-57243-214-7. Johnson, Rafer (2009). Great Athletes. Salem Press. ISBN978-1-58765-473-2.