Horst Marquardt was born to Christian parents in Berlin, Germany, in 1929. He was four years old when Hitler came to power in Germany. When he was ten, his parents moved to Breslau, the capital of the province Silesia (Silesia became a part of Poland after World War II and Breslau became known by its Polish name of Wroclaw). He received his grade school education in Breslau and joined the Hitler Youth. He was fifteen when the Russian armies occupied the Breslau area. His mother and brother fled, but he joined a para-military corps designed to protect the city. Marquardt escaped from Breslau just prior to its being closed, and was one of the last to get out before the city was nearly destroyed. He met his family in mid-Germany and there completed his public school education.

With the fall of Hitler, Marquardt attached himself to the victors in East Germany, and soon joined the Communist Party, working as a journalist on the Party's behalf. He was converted to Christianity through the private reading of a New Testament; after his conversion, he realized he could not continue in the employ of the Party, and left his position to take a job with a Methodist congregation in Berlin. He later studied theology in Frankfurt and was ordained as a minister, working in Berlin and Vienna.

While he was studying in Vienna, the Hungarian Revolution of 1956 occurred, and Marquardt was heavily involved in relief work for the refugees that came into Austria. In the late 1950s, he was given an opportunity to utilize his long-standing interest in radio when he was invited to join a group of men endeavoring to form a German branch of the newly-established Voice of Tangiers (now known as Trans World Radio). This German branch of TWR was officially founded under the name Evangeliums-Rundfunk (which translates "Gospel Radio"), and Marquardt became its director.

Marquardt and his wife Irene were the parents of four children: Christiane, Ulrich, Susanne, and Bettina. They were members of the Evangelical Methodist Church, where Horst served on several national boards. He authored three books: Tips for Radioprediges, Wer Bin Ich, Herr?, and Sprache der Graber. He founded and served as chairman of IDEA, the Information Service of the Evangelical Alliance in West Germany.