Maag described his association with Wilhelm Furtwängler to be the most important in his life. He performed as pianist in a Furtwängler concert with Beethoven's Piano Concerto No. 4 in G major. As recounted by Maag, Furtwängler told him: "Why don't you try to conduct? I have observed you while you were playing the concert tonight, staring at the orchestra more than at the keyboard. It was more you than me that have given the entry." He followed Furtwängler's advice to start out at a small theatre. His new career began as répétiteur and then director at the Swiss Theater Biel-Solothurn from 1943 to 1946. After the first season at Biel-Solothurn, he served as an assistant to Furtwängler prior to the opening of his second season there. After Biel-Solothurn, he became Ernest Ansermet's assistant with the Orchestre de la Suisse Romande.
In 1962, Maag temporarily abandoned his musical career. Believing he was losing touch with music and theology, he sought guidance first from the Greek Orthodox Church and then planned to spend a few months at a Buddhist monastery near Hong Kong. "I decided it was time to retire because I was having too much success," Maag said. The planned "few months" grew to over two years. "Those two years spent meditating and praying in a small cell purified my soul." Maag’s retreat proved to be a detriment to his career when he returned to the world of conducting.
He began recording for Decca Records in October 1950 with the Orchestre de la Suisse Romande. These monaural recordings included Mozart's Symphony No. 29 and Symphony No. 34 and Serenade No. 9 "Posthorn". Maag's early stereophonic sound recordings for Decca were well received, and many have remained in the catalogs for decades. The original LPs, particularly with the London Symphony Orchestra, have become collector's items. He began recording with the London Symphony Orchestra in February 1957. This included a series of Mendelssohn and Mozart works, and the Schumann Piano Concerto. Maag’s favorite recordings came from this association: Mozart’s Symphony No. 29 and Symphony No. 34. His recording from this period of Mendelssohn's Symphony No. 3 was widely admired, and Maag came to be particularly associated with the piece. Maag also recorded two LPs with the Orchestre de la Société des Concerts du Conservatoire in November 1958 with works of Chopin, Delibes, and Rossini. For Decca he also recorded Verdi's Luisa Miller with the National Philharmonic Orchestra in June 1975 and Paer's Leonora with the Bavarian Symphony Orchestra in June and July 1978. With the Berner Symphonie-Orchester of which he was Music Director, Maag made recordings for Innovative Music Productions in the late 1980s. They were licensed to MCA Records for sale in the USA.
Maag's first wife was Jasmina Božin, a stage designer from Bosnia-Herzegovina. Maag was survived by his second wife, Marica Franchi ; a son Georg, a writer of children's books, and a daughter Costanza, from his second wife.