Sandmeyer was born as the last of seven children and attended school in Aarau, studying to become a precision mechanic.

His friend, J. Gustav Schmidt, studied chemistry at the Polytechnikum of Zurich (ETH), and their cooperation in conducting experiments led to Sandmeyer's close contact with chemistry.

In 1882, Sandmeyer was made a chemistry lecturer at the ETH by Viktor Meyer.

When Meyer moved to the University of Göttingen, Sandmeyer followed, but then returned to Zürich after a year to work with Arthur Rudolf Hantzsch.

Sandmeyer was involved in the development of several dyes and invented a new synthesis for indigo.