After pitching at the University of Portland, Dayley was selected in the first round, as the third pick overall, of the 1980 amateur draft by Atlanta Braves.

After spending most of the rest of that season in the minor leagues, he made it to the majors to stay in 1985, where he was converted to being a full-time relief pitcher.

In Game 6 of the 1987 World Series, With the Minnesota Twins already leading 6–5 in the sixth inning, Dayley was brought in to face left-handed hitter Kent Hrbek with the bases loaded and two out.

Ken made eight relief appearances in 1991, but his season was cut short by injuries, most notably a bout with vertigo from which he never really recovered.

Pitching for the Albuquerque Dukes, he appeared in nine games, giving up 14 hits and 12 walks in just 10.1 innings, before being released on June 22, ending his professional career.