He was a third baseman (1021 games) and first baseman (339 games) for the most part, although he did play 10 games in the outfield, 39 games as the designated hitter and 27 games at second base and short stop combined, in his 1397 career starts. He played for the Cincinnati Reds (1974-1981), Houston Astros (1982-1984), New York Mets (1984-1986), Baltimore Orioles (1987), and the Detroit Tigers (1988).

After replacing the exiting Pete Rose at third base in 1979, he wound up on the World Champion Mets team of 1986.

While he is notable as being a very quality hitter, making the Top 10 leaderboard in batting three times, he is mostly known for his role in the 1986 World Series, when he not only scored the winning run in Game 6 on Mookie Wilson's famous grounder through Bill Buckner's legs, but he also hit the tiebreaking home run in Game 7. He went on to win the World Series MVP award, and then became the first player to join a new team the season after winning the award, signing with the Orioles in 1987. His all-time statistics in the postseason include a .279 BA, with 1 home run and 7 RBIs.

In 1986, he also won the Babe Ruth Award for the National League and The Sporting News Comeback Player of the Year Award.

He was also a winner of the Hutch Award in 1983.

In a 13-season career, he put together a .271 batting average with 84 home runs and 595 RBIs in 1495 games. He had 490 runs and 14 career stolen bases. He accumulated 266 doubles and 1311 hits in total, in 4829 at bats.

In his managerial career, he is 125-137, managing the Reds from 1996-97 and in 2003 for 1 game. In 1997, he forgot how many outs there were in a game and called for a bunt at an inopportune time. He later fined himself 250 dollars for the incident. The team's lack of success would lead to his eventual firing and his replacement with Jack McKeon.

Knight is married to golf star, Nancy Lopez, and briefly caddied for her after retiring.