In the 1988 he moved to Bellingham, Washington to pursue a baseball career for the Class A Seattle's farm team. He had trouble in the family, include the fight with his father and bad game for a new team, ultimately all this has led to suicide attempts. Ken survived and in 1989 he joined the Seattle Mariners team. He became the youngest professional player in the major league being at the age of 19.

Playing for the Seattle Mariners, Ken became the one of the best all around hitters and fielders. During his career there he hit more than forty homers six times. In the 1997 he was honored with an American League MVP title as well as won ten straight Gold Gloves. Previously, in the 1995 game against the New York Yankees, he scored the winning run in Seattle's five-game victory over during the American League Division Series.

In the 1999 he decided to leave from the Marines, because of family reasons. He wanted to be closer to his family in Cincinnati. In the 2000 asked the team to trade him to the Reds, so in he same year he became a free agent and chose to be traded to Cincinnati. He moved back to his native land and there he met the line of problems, include several injuries over the past three seasons.

 In 2004 he becomes one of the top power hitters in baseball. In the same year Ken became just the 20th player to hit 5 hundreds career homeruns. Afterwards, on expressed that he would want to play for Seattle team again if the opportunity will comes to him. His last game for Reds team was on July 30, 2008.

In the 2008 he joined to the Chicago White Sox team with a one year contract. He made his first home run being a player of the White Sox on August 20, 2008. In the next year he became a team member for the Seattle Mariners again.

During his successful career, as an outfielder, he won four American League home run titles, ten gold gloves as well as seven silver slugger awards. He was elected to the Hall of Fame with his jersey number 24.

In his childhood he was interested in music and even took piano lessons. He is married to Melisa Griffey. Nelson and his wife are blessed with three kids, two sons, Trey Kenneth and Tevin Kendall, and a daughter, Taryn Kennedy. His son Tevin Kendall is an adopted kid. A famous American player has an estimated net worth of that whopping $85 million dollars and also earns a massive salary of 2.3 million.