His father was the Federal Councilor, Emil Welti and his first wife was Lydia Escher, a major patron of the arts.

He received his doctorate in law from the University of Bern in 1880 and moved to Winterthur, to work for the Swiss Accident Insurance Institute.

He eventually sat on the Board of Directors there and at several other companies; notably the Schweizerische Mobiliar [de]; serving as its President from 1904 to 1937.

When this was discovered, she and Karl fled, but Welti's father intervened, had Stauffer arrested and Lydia placed in an insane asylum.

After that, he began a four-decade long collaboration with the historian, Walther Merz [de], doing research on behalf of the Collection of Swiss Law Sources and studying town privileges.