Undrafted by both the NFL and the fledgling AFL out of TCU in 1960, Sherrill Headrick signed with Lamar Hunt's Dallas Texans and quickly became one of the fiercest defensive enforcers in professional football. Earning the nickname "Psycho" for his reckless disregard for his own body and staggering tolerance for pain, Headrick anchored the center of Hank Stram's middle defense from the very first snap in franchise history.

Headrick hit the ground running with an unprecedented three-year run of dominance to open his career. He captured First-Team All-AFL honors in each of his first three professional seasons (1960, 1961, and 1962), combining brutal tackling force with keen pre-snap diagnostic skills. His signature moment of toughness came in the 1962 AFL Championship Game, where he played through a fractured vertebra suffered early in the contest to lead Dallas to a historic 20–17 double-overtime victory over the Houston Oilers in the franchise's final game in Texas.

When the organization relocated north to become the Kansas City Chiefs in 1963, Headrick remained the defensive unit's undisputed emotional leader. Though severe injuries—including broken bones, torn ligaments, and a dislocated shoulder—accumulated over the years, his standard of play rarely dipped. He earned two additional Second-Team All-AFL selections in 1964 and 1966 and collected five total AFL All-Star nods during his tenure.

In 1966, Headrick anchored the linebacking corps alongside EJ Holub and Bobby Bell, helping Kansas City win its second AFL title and reach Super Bowl I against the Green Bay Packers. His knack for creating takeaways in key moments netted 14 interceptions and 12 fumble recoveries during his eight years in red and gold before he played a final active season with the Cincinnati Bengals in 1968.

With the Chiefs, Headrick netted 14 interceptions and nine sacks over 108 games.  He entered the Chiefs Hall of Fame in 1993.