Nicknamed "Moose" by former teammate Babe Laufenberg, Johnston became a full-time starter in 1991 and was a key member of the Cowboys to three Super Bowl winning teams (1993, 1994 and 1996). A two-time Pro Bowl selection (1993 and 1994), he played in 149 consecutive games, including postseasons, before sitting out a game on October 19, 1997 against Jacksonville with a career-threatening neck injury. Johnston returned in 1998 to play in 13 games, but just one game in 1999 before neck problems sidelined him for the rest of the year. He finished his career catching 294 passes for 2,227 yards and 14 touchdowns, as well as 232 rushes for 753 yards and eight touchdowns. He had a career-high, 50 receptions in 1993.

Johnston has contributed time to many charities including the Special Olympics, Children's Cancer Fund, Cystic Fibrosis and Literacy Instruction for Texas (LIFT). He has served as honorary chairman for MDA's "Aisles of Smiles" campaign and for the American Cancer Society's Gunslingers' Ball. In 1999, Johnston's numerous community service activities led to his nomination for the prestigious "Whizzer White Humanitarian Award."