He was a law clerk to former Associate Justice Tom C. Clark of the Supreme Court of the United States while Clark had senior status and was a visiting judge on several United States Courts of Appeals from 1976 to 1977.

He was in private practice in Omaha, Nebraska from 1977 to 1980, then in Minneapolis, Minnesota until 1981, and then in McPherson, Kansas until 1996.

On October 18, 1995, Marten was nominated by President Bill Clinton to a seat on the United States District Court for the District of Kansas vacated by Patrick F. Kelly.

[2] On August 15, 2013, Judge Marten held that an abortion opponent's letter to a Wichita doctor stating that someone might place an explosive under her car, absent sufficient contextual evidence indicating an imminent threat, is constitutionally protected speech and not a "true threat" under existing law.

[3] Judge Marten noted that the government supplied no evidence that actual violence against Dr. Mila Means was likely or imminent, especially since after receiving the letter the doctor changed plans to provide abortion services in Kansas.