Brian Mayenfisch was born in 1991. He studied Law at the School of Law of Fribourg, where he obtained his Bachelor of Law in 2013; he then continued his studies at the Scool of Law of Fribourg and the Universität zu Köln (Germany), before receiving his Master of Law in 2015.
In 2016, Brian Mayenfisch joined the Swiss Federal Administrative Court (St. Gallen), starting as an intern and then as a law clerk.

Brian Mayenfisch joined 100 Rhône Avocats in September 2019, firstly as a Junior Associate, and then as an Associate in December 2021, after being admitted to the Geneva Bar the same year. His main areas of practice include administrative law, construction and real estate law, environmental and energy law, contract law, and administrative and civil litigation.

Finally, Brian Mayenfisch co-authored an article for the French-speaking Commentary on the Swiss Federal Constitution (Gregor T. Chatton/Brian Mayenfisch, art. 115), which was published in June 2021 by Helbing Lichtenhahn.

He is bilingual in French and English and has a good knowledge of German.