He played college football for the Delaware Fightin' Blue Hens and was selected in the 11th round of the 1967 NFL/AFL draft by the New York Jets.
[3] After not playing as a freshman in 1963, Slattery won a starting role as a lineman to begin the 1964 season, the first sophomore to do so on the line for the team since 1954.
[7] He remained a starter for his subsequent three years with the Fightin' Blue Hens and became one of the top linemen in school history; during his career, he was the only person for Delaware to start on both sides of the ball, being a standout tackle on offense and linebacker on defense.
[10] He was the heaviest player in the starting lineup – as well as the second-tallest – but was still regarded as one of the fastest members of the line, as well as, according to several coaches, "probably the team's best lineman–both offensively and defensively.
[11][4][13][14] He was additionally named the co-winner of the Delaware Athlete of the Year award, along with Philadelphia Phillies pitcher Chris Short, as selected by the Wilmington Sportswriters Association.
[15] He also spent several years as a member of the school's track and field team, competing in weight events, although he skipped participating as a senior to focus solely on football.
[19][20] He later briefly joined the Waterbury Orbits of the Atlantic Coast Football League (ACFL) in September, although he is not known to have played in any games.