The Welser family was an old patrician family of Augsburg (southern Germany) and one of the wealthiest in Germany. His uncle, Bartholomeus had been an advisor to the Emperor Charles V and is said to have lent him twelve tons of gold. In 1528 Bartholomeus sent a fleet to the New World and established a colony in Venezuela, which was taken over by the Spanish in 1555.

Marc Welser was sent to Rome at the age of 16 and became a very fine scholar of Greek and Latin; he also became fluent in Italian and studied antiquities. Upon his return to Augsburg, he became a lawyer and in 1592 became a member of the Senate of that city. He was elected the Senate's Council. His passion, however, was history, antiquities, and philology, and he corresponded on these subjects with the foremost scholars in Europe. He published books on the antiquities of Italy and Augsburg, on martyrs of the early church, and early German history. He also prepared an edition of Emperor Frederick II's (13th century) book On the Art of Hunting with Birds, and published several editions of hitherto unpublished Greek sources.

Among Welser's correspondents were a number of Jesuit scholars, such as Christoph Clavius. It was Clavius who assured Welser that Galileo's telescopic discoveries were real. At the end of 1611, the Jesuit mathematician Christoph Scheiner, wrote three letters on sunspots to Welser, and Welser published them early in 1612 at his own press. He sent Galileo a copy of the tract asking for his opinion. Galileo's responses and Scheiner's second tract on the subject were published by the Lyncean Academy in 1613 under the title Istoria e Dimostrazioni intorno alle Macchie Solari e loro Accidenti ("History and Demonstrations concerning Solar Spots and their Properties"). Welser was elected at this time to the Lyncean Academy. After a long and very painful battle with gout, he died in 1614. His collected works (the introduction to which is the source of virtually all information about his life) were published in 1682.